Why don't all pilots receive at least basic instrument training?
While the number of pilots who die flying VFR into IMC is tragic, it's a miniscule percentage of pilots overall. The vast majority of us take the risk seriously and either stay clear of bad weather or get the additional training to be able to handle it safely. The former have no need for the additional training, and the latter are already getting it.

Every pilot in theory knows how to fly by instruments. The problem is that it is surprisingly difficult to believe them when your body is telling you they're wrong. And once you spend a few hours training yourself out of that problem, you still have to spend dozens more hours learning how to fly in the IFR system, which is an entirely different beast than flying VFR--and to many (perhaps even most) pilots not of interest at all.

Also, flying is a perishable skill. Having flown IFR once years ago doesn't mean you can safely do it today, especially if it happens unexpectedly. Worse, having had training may give people a false sense of confidence in their skills, leading them to knowingly fly into bad weather that they would have avoided if they'd never had that training.

FWIW, using your logic, the FAA does now require a few hours of hood time to get a PPL, but the purpose of that is to demonstrate to students how they are not prepared to handle it, other than to make an emergency 180 and return to safety. It's like how students practice stalls so they know how to recover from (and ideally avoid) them, not to encourage pilots to stall their planes in normal flight. And the stats haven't changed.

In the end, the root problem isn't the lack of instrument training; it's poor judgment. Take away one way for high-risk pilots to kill themselves and they'll just find another, so nothing changes except the cost of training went up for the vast majority of pilots who didn't need saving in the first place.