This really isn't true, though.
Mario 3D platformers (specifically 64) and Mario Kart both had inspired plenty of copycats, much to the point that both Sega and Sony have both made multiple attempts at chasing Mario Kart's success, with Sega having three separate series of Mario Kart clones just based around sonic (and that's not including Sonic & Sega All Stars) and Sony having done kart racers for Crash, Jak, and LBP. Mario Kart clones have obviously tapered off, but we are clearly still seeing some companies sticking with them (with some, like Sonic & Sega All Stars, actually being successful). Mario 64 is still inspiring clones, albeit largely via licensed games.
Smash clones actually were a thing throughout the 2000's, and Sony obviously still wanted to follow it as of a couple years ago.
Even New Super Mario Bros inspired a resurgence in mainstream 2D platformers.
The Halo comparison actually works even more under this light, because it did inspire tons of copy cats, and those have now tapered off as devs are chasing other stuff now despite Halo still being a successful franchise.