> Why would a shitty app promote more sales?
Because part of it being shitty is obnoxiously advertising you financial products, making it hard to perform the tasks you want with your money without being upsold something, and making it near-impossible to use your account without going through the app/webapp (or a visit to the bank, or the ATM - which is also pushing ads).
The only reason I use MegaBank and its obnoxious but functional app is that Local Credit Union can't afford to build a decently functional app.