Two reasons.
one. basically, it's the easiest method and is the existing method (more important things to change at the immediate present, i.e. terminals/bugfixes/lf tweaks)
two. If you don't create a way to get a plane relatively immediately, a lot of new players will be immediately bored and never return... so until we come up with a good means to either select amongst a few used aircraft, or something else otherwise immediate, we're sticking with the 1 free plane method.
EDIT: IT is, though, in the long term goals. and will likely be in for the age after next, if all goes to plan.