Is someone irresponsible enough to pursue causal, unprotected sex with random partners also likely to be responsible enough to visit a doctor proactively and comply with a prep regimen?

Anecdotally, very much so - at least among the gay/MSM community.

The thing is, people generally quite like having sex. That comes with some level of risk, which can be mitigated in various different ways - and PrEP is one of those tools.

All humans engage in some level of risky behaviour; evidence suggests that moralising about “responsible behaviour” or “self control” is rarely effective at mitigating that risk.