Secondly because no ship he could possibly build would be as fast as Ring FTL. Or have any added utility as the Ring sees to his comfort and has subspace pockets.
Speedwise, I don't think it's either-or, unless there's an arbitrary weight limit to Ring FTL. I'd imagine it's more a matter of power drain, which then becomes a comfort vs. efficiency thing (and with his power battery right there, not as important).
Ring travel is great if you just care about getting from point A to point B as quickly as possible. If you're basically a homeless space adventurer, however, having a portable home base seems like it would be useful.
Even assuming for the sake of argument that Paul
could live out of a ring-generated bubble and a subspace suitcase for as long as he's off Earth, there's the possibilities of:
- him being sick, injured, unconscious or otherwise in a condition where shelter he doesn't have to maintain would be useful
- being in a situation where using emotional spectrum energy isn't conducive to flying under the radar
- being unable to recharge or otherwise needing to ration power usage
- gaining some possession that he can't/doesn't want to trust to subspace
- obtaining an ally/prisoner/someone else that he wants to be able to keep protected while off somewhere else doing other stuff
... all of which seem plausible or are things that he's literally already encountered.
I'm honestly a bit fuzzy on what his plans are. If the idea is that he's going to leave Earth tomorrow and be kneeling in front of the Controllers ASAP, then he should just Ring there.
If, instead, the idea is to look up Adam Strange or rescue Starfire or go take a peek at the remains of Krypton or any other sort of 'space adventure' sort of things, then a ship might be nice, so he's not basically living like a vagrant.
I never quite got the Green Lantern thing, where someone apparently had the fridge logic moment of "OMG, why was Abin Sur travelling in a spaceship in the first place?" and felt forced to come up with explanations for that.
To me, if you're a Lantern who's doing his job right, it's a full-time job where you spend the overwhelming majority of your time not at home. And, as anyone with an actual job like that knows, being a homeless person living out of a suitcase is a PITA. Makes perfect sense to me that you're going to have a shuttle or something you can relax in at the end of the day.