If they have time to write SQL queries, they have time to write stored functions.
Its really not that difficult and it certainly does not take a substantial amount of time.
They have the time to write SQL queries in their code
They don't have time to (or better, shouldn't) materialize them as a stored function in the DB
"Oh but your CI/CD should automatically..." Let me stop right there
The time they spend with this can be better used to ship and to improve their SW to customers, not with yak shaving
Which is why they end up spending time on mea-culpa "we take your data security seriously, but clearly not seriously enough" emails when they inevitably get pwned by a completely predictable and avoidable SQL injection attack.
The sort of startups you describe are jokes that barley take security seriously, let alone know what a pen-test or code audit is, let alone actually do them on a regular basis.