While GM decides what monsters to throw into players, they still need to know what they could use without it being either underwhelming or overwhelming. You dismiss this simply by saying: "just be a good DM".

• New DM's will want guidelines to start from.
• If combat is important having written rules help to use consistent ruling on same situation in different instances.
• Story focused DM might reduce amount of effort needed to plan combat, since there is no need to build it from scratch.

Disadvantage of having to look up rules then you don't remember them could be mitigated by just saying: Look guys, I don't remember ruling now, so not to break the flow, I will rule it this way, and look it up later.

So while for most players rule heavy systems are less accessible, they are actually more accessible for many DMs, and since mastering have much higher barrier of entry, such systems at least should not be dismissed outright.