Simply because Bitcoin was decentralized and that what makes this coin so powerful without control of anyone. If this such this wallet exist, there's no doubt that this will be popular and will be more trusted by everyone. However, this will be kind of easy to control, this will not become decentralized and tons of trusted wallet might close that could possibly affect the crypto world.
there are a lot of wallets here in the industry and those are wallets that no one are controlling. If the bitcoin developers make their own wallet then probably they are the one who has control over those wallets.
I think mate those who own the system have control over it. Well, technically not entirely because you have control over your passphrase, pins, and such but remember, believe it or not, they are the developers of the system so they have full control over it.