"I don't fully understand why more effort is not put into a hardware solution, where roads are simply marked up for self-driving vehicles, e.g. magnets lining the lanes or something like that."

Volvo is plugging for magnets in roads in Sweden, so they can find lanes in the snow. It might happen, because it can also be used for snowplow guidance. In heavy-snow areas, posts, poles, and even over-road arrows (Japan uses this in Hokkaido) are placed to provide guidance. It's not intended to replace vision and LIDAR, just as additional guidance hints for bad conditions.

A simple RF transmitter in new versions of the existing reflector discs would do it.

It wouldn't need to do anything but "ping", the battery could last for years. You could even have a mechanism to disable it when the lines are repainted, though normally roads that use the existing version of these reflectors would have them removed when moving lines.

The transmitter would have to be powerful enough to go through a small amount of snow in some states.

The question is can we get the car companies to agree on a standard for this? Would it make sense to put it in the frequency range allocated for vehicle radar systems?

What we get instead is an attempt to define the whole system de jure, including full V2V protocols and specifications.