Because generally exchanges are rather insecure comparing to other wallets like Electrum or Coinomi. It could be hacked or might also be a scam, you can't be
Where did you get this idea? Electrum and Coinomi do have their own feature on which Coinbase or any exchanges didn't have. Vice-versa, Coinbase, and any exchange also have their unique feature that hardware wallets, desktop wallets don't have.
Regardless of features, non-custodial wallets (like Electrum and Coinomi) are generally considered safer because they have fewer attack vectors (more resistant to phishing, don't have to worry about password leaks, etc.). They don't have the capability to hold your funds for any reason either.
Coinbase
does have the advantage of insuring their customers' funds (so they might actually be better than a lot of exchanges for storage), but you won't really need insurance if you know how to take care of coins.
Of course, and I think everyone already knows that Electrum and Coinomi are generally much safer compared to any centralized exchanges out there even how reputable it is when we are talking about storing our own coins.
What I'm pointing to my statement there is not about the security between those wallets but where did OP got the idea that exchanges are insecure to those non-custodial wallets.
OP is telling anyone not to use Coinbase to store their coins which is already a general and basic rule, guidelines and a must thing to do. Why use Coinbase for that purpose? Just use their features instead.