Spoiler: Because it is very important to the BTC team's hope of keeping BTC's value up and to supporting their attack on the real dream of Bitcoin. They know BTC's best days are numbered if BCH succeeds in fulfilling the Bitcoin dream. They are desperate to keep fooling the public to keep their house-of-cards from collapsing.
If you follow my writings here you know I believe BTC was captured and intentionally changed into something that no longer works towards fulfilling the real dream of Bitcoin (sufficiently decentralized peer-to-peer electronic cash for the world's people). That is sad.
Since most all activity is allowed in the permissionless crypto space (unless it violates an enforceable law like fraud, theft, etc), we pretty much must accept and learn from this sort of attack on the Bitcoin dream. BTC is lost and those who believe in the real dream of Bitcoin must move on. Many did move on and the Bitcoin Cash projects (BCH, BCHA) are the only Bitcoin projects I believe are still attempting to make peer-to-peer electronic cash for the world's people.
Mad Bro?
I believe most BCH fans probably do not mind the ongoing existence of BTC. For many of us it was our first crypto-love. For now I have no problem with BTC following it's new path. Because BTC's team has abandoned the dream and intentionally limited it's growth potential, I do not even see BTC as a real competitor to BCH. If BCH develops scaling that allows for massive world wide adoption, I believe it will go viral and grow orders of magnitude larger than BTC. BCH would take most of the security (hash power) and grow in value/use so much larger, I think BTC would become just a footnote in the history of cryptocurrancy. I expect BTC would retain a significant collector's value and never die. So I have no problem coexisting with BTC itself.
Admittedly, it is hard for many to forgive the new owners of BTC since they stole our beloved project, centralized control of it's development team and corrupted it to achieve their dark intentions. Their hidden intention appears to be to block the real dream of Bitcoin (rather than the many other stories like Blockstream greed they tell). So, there is good reason for anger on the BCH side. Even so, I think the BCH community would be happy to move on and ignore BTC since it is not a real competitor.
BTC cannot allow BCH to succeed
BCH does not really care much about BTC's small future, but the new owners of BTC development must stop or delay BCH (or any real Bitcoin project) for at least 2 reasons:
1) The forces that captured BTC are anti-Bitcoin. They captured BTC to stop/delay the dream of Bitcoin and BCH is still working to fulfill that dream
And
2) They know our success would lead to their "fake" Bitcoin's collapse. A successful BCH would grow so large and well used that BTC would seem small and useless in comparison. BCH would gain most of the security (hash rate) and become much more valuable. BTC's price would fall and it would become a collectors item rather than a reliable store-of-wealth (which it isn't).
They know this, so they must stop or delay any real attempt to fulfill the dream of Bitcoin. To fool the public about their intentions and to slow BCH down, I believe the BTC team has implemented massive social-engineering efforts going 24/7 to fool people into believing their lies. This effort is really just a continuation of what has been going on for years and was used to help them capture BTC from the real Bitcoin community years ago.
Apparently a lie oft-repeated can become like truth
As I see it, the professional and dishonest social-engineering spreads many small and large lies mixed with truth in a web of deceit blanketing most crypto-related social media. Stories like being a reliable store-of-value and BTC still intending to be a real Bitcoin for the people of the world are some of the big lies. It turns out professionally implemented dishonesty works well at fooling people if you use many accounts and keep it up long enough. Especially if the dishonest messages go un-challenged. So, we must stand up to the dishonesty or things get worse. They know that and they keep at it wearing down the truth tellers who are often just volunteers and can only do so much against their seemingly massive and well funded efforts.
Their methods are often very complex and sophisticated. Their professionally crafted stories often sound very nice and believable. They do things like pretend to be BCH fans and suggest we stop standing up to their lies with posts like: 'Why all the fighting? Can't we just exist side-by-side and live happily-ever-after'. This strategy is also used to make it seem like BCH fans are the ones causing the "war" to continue.
Intentionally bad vibes
I agree the divisiveness of standing up to the dishonesty is bad for the community vibe. I believe that is another goal of their regular posts on r/BTC. R/BTC is the uncensored Reddit Bitcoin-discussion place and they can't censor the truth there (like they do on other important reddits). So they lie repeatedly with large numbers of accounts and make the discussions they do not like toxic to turn new visitors away. Then they pretend to be offended and ask "why all the toxicity?", lol. Their tactics are quite effective and there is only so much much we can do.
Peace?
I think the real Bitcoin community would like to have PEACE. We hope they will stop lying about BTC and BCH so that can happen. If their position was as strong and well-intentioned as they pretend, there would be no need for all the dishonesty and the war could end. Sadly, they know their position is weak and can only be kept up by lies and market manipulation.
I see your point, I will think about.