It's 30-40% gasoline because that is what the refining process has been optimized to produce. You can get other hydrocarbon mixes by choosing different process parameters.

Also, a huge part of oil refining is already shortening the chain lengths by adding hydrogen and "cracking" the hydrocarbon molecules [1]. They don't do that to gasoline at the moment because it has such a huge available market that it doesn't make economic sense to process it further. If that market disappears however, then we absolutely would be able to convert "waste" gasoline further into jet fuel or plastics or whatever.

[1] https://en.wikipedia.org/wiki/Cracking_(chemistry)