Perseverance will land using terrain-relative autonomous navigation, demonstrating fairly precise landing. There are multiple reasons to split the mission. Per

Perseverance will land using terrain-relative autonomous navigation, demonstrating fairly precise landing. There are multiple reasons to split the mission. Perhaps the main one is that ~1 ton is approximately the upper limit of existing Mars landing technology which is enough for a powerful rover like Perseverance that can carry several sample return canisters (and live long enough to find really good samples) OR a rocket capable of returning the samples back (with just a small, basic canister retriever rover). Yeah, you could just put a sampling arm on the rocket lander, but that's a lot of wasted opportunity (heh). Perseverance should have plenty of range, and the autonomous landing navigation tech should have sufficient precision that surface rendezvous should not be a major challenge.