Exactly. RADAR has exactly the same problem that LIDAR does in terms of removing ground clutter. Is that a bridge that you're about to go under or a trailer that you're about to go under? Does the road slope down just ahead or does the road turn right before a sheer cliff? Is that a bump in the road or a body? And so on.
The only way to be sure is with vision, and maybe not even then.
That doesn't really help much. I mean yes, it can help a little bit, but ultimately the purpose of either RADAR or LIDAR is to identify additional areas of concern for the vision system to look into, so resolution isn't particularly important unless you're trying to use it to avoid running over small animals in the road or something. For the most part, interesting obstacles are not small, and small obstacles are not interesting.