Rational individually, slow collectively
A thorough pre-shot routine genuinely improves a single player's chance of hitting the shot they intend, so from that player's perspective taking extra time is a reasonable investment. The problem is that this logic applies to every player simultaneously, and a field of players each behaving individually rationally produces a round that takes far longer than anyone would choose if pace were the only consideration, which is a familiar kind of collective problem.
Enforcement without real teeth
Most tours have formal time limits and warning systems for slow play, but penalties are applied rarely and only after a lengthy process of timing and warnings, which makes the consequence feel distant and unlikely rather than a genuine deterrent. A rule that is technically on the books but almost never enforced strictly does little to change behaviour, since players can reasonably judge the risk of an actual penalty to be very small.
The group timing problem
Timing is typically applied to a group rather than an individual, since separating out which specific player caused a delay is difficult in real time. This lets a genuinely slow player hide within a group that includes faster partners, and it removes much of the individual accountability that would be needed to change one player's habits directly, which is why the same names keep appearing in slow-play discussions year after year.