Yes they can. Saensak Muangsurin became boxing world champion in his 3rd fight. Do you attribute that to the little training he would have done in boxing, or the hundreds of fights in Muay Thai he had? Lomachenko who had 400 amateur boxing fights, trained in pure boxing from childhood, could only equal that record. Current boxing world champion Knockout CP Freshmart became world champion in 9 fights. Once again his muay experience obviously contributed heavily to his success.
Regarding "why are there no top 50 ATGs who are kickboxers?" -Simple demographics: The biggest boxing markets are USA and Mexico, neither of which are huge KB countries. The fact that Klitschko and Khaosai Galaxy did become ATGs despite not being from America or Mexico makes it all the more impressive.