> Why do they “stack up” when one DCA’s?
You can send to one address and grow the UTXO
> And how does that affect one’s wallet or bitcoin in the future?
Address reuse reduces privacy.
> I can’t understand what the problem is with DCAing and UTXOs.
Not sure what problem you are referring to. Check out https://river.com/learn/bitcoins-utxo-model/