I think because H2 is so common. Given that and how gravity works, let's look at the possibilities:

small thing (say up to size of smaller planets): only solids can clump together and stay together, gas would just fizz away

medium thing (earthish to gas giantish, say): solids clump together and that provides enough gravity (and sometimes magnetics) to capture/keep some gas too

large thing (star sized): you can just capture everything around. Sure that'll contain some metal, but since most of what's out there is H2 and you're gulping it all up, you end up mostly gas

Which planets? The big ones are mostly H2. The small fry with weak gravity lose theirs.