The kinds of things that the Monero developers and community have built could be on the same world changing scale as the internet. I think I'm preaching to the choir here so I'll leave it at that. Why then is everyone still using Venmo? Why can't I pay for my groceries with Monero? It's because adoption is hard; and by having two apps with almost identical function we're making it even harder to onboard new users.
2. It's confusing to everyone.
Cake Lab's great site, monero.com shares a name with this app. In a recent tweet they announced many other currencies are now available to trade on monero.com. From the context of the tweet alone there's no way to tell if this feature has been brought to the app monero.com or the website monero.com.
3. Time developing and promoting needs to split between Cake and monero.com wallets.
Stuff like this wastes the time of the people at Cake Labs. Stuff like porting over new features and fixes days after they were available in Cake Wallet is a waste of Cake Labs time. This split in effort will mean less eyes out for critical security flaws. To the people who say the monero.com app has less of an attack surface, well now with two apps we've essentially doubled the attack surface. Plus, your phone is an electronic Benedict Arnold that fits in your pocket. Any benefit that you might get from the reduced attack surface in an app is negligible compared to the risk from the humongous attack surface that you carry around all day.
4. There are plenty of Monero only wallets available already.
I believe interoperability is the future. Everyone should be able to buy Monero from the open market because it offers all those things we like (again preaching to the choir). But, if you prefer something that only supports Monero then there're already other top quality options available like Monerujo and MyMonero. Do we really need another Monero only wallet option?