Well, new is relative. The AR design dates from the middle of the last century. Original ARs and M-16s from the Vietnam era had a permanently attached handle. Mounting optics wasn't a priority.
The NEWER ARs can take a scope. FALs get a replacement dust cover. It's no different than replacing a smooth fore-end with a one with rails.
As hiredhand can attest (at one time we both had FALs) a good dust cover doesn't come off and works wonderfully.
I guess you weren’t around to see tons and tons of scopes mounted on carrying handles in the 1970s, or scopes mounted on home-made flattops which only required cutting off the carrying handle, which is not integral to the receiver but simply a cast on appendage.
The point being, an ARs upper receiver is inherently stiff and ready for very accurate iron or glass sights.
For an AK or a FAL, you have to do some pretty serious aftermarket purchases and work to put on a solid optic. It’s just inherent in the design of the rifles with dust covers.