It's all about tradeoffs. You could increase the number of pilots to hopefully reduce the number of these incidents, but the added complexity might increase other incidents and would reduce the capacity for paid passengers and increase the labor costs when there's already a shortage of qualified pilots willing to work for the airlines.

Same thing with the doors. They got strong locks and protocols to use them in response to rare events, but it makes other rare events harder to manage.

We generally live in a pretty trusting world, which enables many things, including violations of trust with major consequences.