If you want to go there, where arbitrary cutoffs make little sense, I have a long list of planes that should be fine to fly under Sport Pilot privileges, but aren't. The 150/152 pilots would probably agree that if my RV-12 is LSA legal, their planes should be... but, no. Obviously the mighty Cessna 152 is far too dangerous.
Some examples are truly ridiculous. You can have two identical Aeronca Champs, with every single part matching as well as every detail of their history. Same engine, same prop, same equipment, landing gear, service history, everything. Totally identical, indistinguishable other than the N-number. One is LSA legal, the other is not and can never be made so, ever.
Sigh... These things don't have to make sense, they just are.
And you're right about the expense, but this is just not a cheap hobby. Probably never has been; I remember it being completely out of reach for me in the early 80s when I could have flown for $20 per hour wet. Some things just aren't cheap.