In-universe it's a combination of A) who's going to go tell the 9ft tall demigod that they're not allowed to exist anymore, and B) Space Marines are uber spec-ops who excel in surgical strikes that cripple enemy armies' leadership that a tank simply can't do, and are therefore a massive force multiplier despite their small numbers.
Out of universe, GW can't imagine advertising with anyone other than SpHeS mAhReEnS or generally acknowledging that xenos exist other than the occasional Necron novel that slaps unreasonably hard.
In-universe it's a combination of A) who's going to go tell the 9ft tall demigod that they're not allowed to exist anymore, and B) Space Marines are uber spec-ops who excel in surgical strikes that cripple enemy armies' leadership that a tank simply can't do, and are therefore a massive force multiplier despite their small numbers.
Out of universe, GW can't imagine advertising with anyone other than SpHeS mAhReEnS or generally acknowledging that xenos exist other than the occasional Necron novel that slaps unreasonably hard.