Although WhatsApp has more users than Telegram, it is nowhere near the popularity of Telegram. As a consequence, Whatsapp is copying Telegram's features to satisfy its users.

Despite WhatsApp having more users than telegram, it doesn't signify that it is better than Telegram. Some WhatsApp users are not willing to migrate to other platforms and others don't leave because their contacts are there.
For a long time, WhatsApp remained a boring simple app but now that the telegram is skyrocketing, WhatsApp wants to copy telegram's features to gain additional attention.

WhatsApp's new feature is a copy of Telegram's

Meta, owner of WhatsApp is preparing an Update for IOS and Android versions of the app. In the new version, reacting to others' messages will be possible. Although right now it is not available for the general users as it is being tested in beta version and there is no info on when this feature will be accessible for everyone.

This feature was added to the telegram a while ago and was welcomed by many users.
Although meta says they are working on this feature for some time, their biggest obstacle has been the layout of reactions that has largely been a copy of Telegram.

from leaked screenshots one can see that the reaction panel in WhatsApp is similar to that one of the Telegram. right now, there are only six emojis for reaction in WhatsApp. According to how we have come to know WhatsApp, probably it will never reach the elegance of reactions in Telegram.