The iconic BMW R75 with or without sidecar had a 26 hp engine. You can make a car with that, like the Kubelwagen, but obviously that's far from ideal for a military vehicle that needs to go offroad. Meanwhile you can have a decent all terrain motorcycle with that power, which being a motorcycle can also go into places car's can't, and be overall cheaper.
For comparison, Kubelwagen's contemporary Jeep MB has 60 hp while being only half heavier.
Some special forces and recon units still use motorcycles, though not with sidecars, they look a lot like motocross ones. During Cold War some motorcycles were also used as recoilless SPG and mortar carriers.
Conservatism of the general and also costs. Early assault rifles were complicated and unreliable by any standard, and especially so when compared to the bolt action rifles of the era.
Similar concerns also limited later mass use of semi-auto rifles, especially German G-43 and Soviet SVT-40.
Weapon and ammo manufacturing efficiency has improved a lot during WW2, and the change in attitude to more complex weapons was at least in part caused by that - they were no longer a 5-10x the cost to outweight their advantages, and ammo was more available too.
Thompson is a great example of such advances in engineering - over the time of WW2, the same, famously complex but also effective (as all the mobsters agreed) was modified to be simpler and better, with over 75% reduction in production cost.
There are more or less complete lists like this to give you a perspective on it.
When you compare the price of a BAR or Bren to mass used rifles, it becomes obvious why these weren't being reengineered into lighter proto battle rifles rifles of some kind, but instead remained valued and never too common support weapons, and why the Sturmgewehr became the first truly mass produced weapon of similar kind.