I would go for an even tougher question, why does it have Wi-Fi at all?

Try putting 4 of them (and their big ears) in 1 1U rack space.

The RM version is fine (limited in power but fine) for some uses, but the HaxD-IN?

Isn't that what the "RB5009UPr+S+IN" offers?

Not sure to understand.

The L009 comes in two versions one (IMHO right) without WiFi/Radio and one (IMHO wrong) with (2.4GHz radio).
For both it is highlighted that 4 of them can be mounted in a 1U space of a rack, for the former it is fine, for the second you will need to use external antennas and in any case it makes no sense to have four radios/eight antennas in a same spot.

What one could do at most in a rack is to have 1 L009 with the radio and 3 without, but the "real" use of a L009 with radio is a self-standing device (and in that use it could be useful to have the 5GHz radio) but there are already more suitable Mikrotik devices for that use (Ax2, Ax3) the only drawback being less ports, easily solved with an added small switch.

In any case, particularly with 5GHz, you need to distribute as much as possible the coverage with more access points, so having the "main" router being also a Wi-Fi device is becoming (IMHO) less common/useful (unless it is a small house/office that can be all covered by the single router Wi-Fi from its position, which often depends on where the ISP cable arrives), it seems to me increasingly common that the router is placed *somewhere* in a corner and Wi-Fi coverage is obtained through access points even in relatively small homes/offices.

The RB009 is a much more powerful device that comes in the same form factor of the L009 (4 can be mounted in i1U) in two models, both without radios, one with and one without PoE out.

Sure, I was making fun at the idea, clearly promoted by the good Mikrotik guys about mounting 4 L009's with radios on a single 1U:
https://mikrotik.com/product/l009uigs_2haxd_in

L009UiGS-2HaxD-IN
L009 is more than just a router. This product line is up to 4 times faster than RB2011, it has a modern ARM CPU with container support, an innovative enclosure that allows mounting up to four routers in a single 1U space, more RAM, PoE, and 2.5G SFP support. This version comes with 2.4 GHz ax dual-chain wireless.

You can, but you shouldn't just because you can.