Slo-pitch absolutely is meant to be an equalizing form of baseball; while slo-pitch hitting and pitching are skills unto themselves, it’s one that can be learned by people who would not be able to keep up with baseball pitching and hitting against the same opponents.
Fastpitch softball, however, is not an equalizing form of baseball. It’s the same thing but on a smaller diamond, except with a ball that’s harder to catch and throw.
The simple answer to the OP is “because.” It’s always been done this way, so it continues to be done this way. There is no particular, logical reason why female players should play with a softball instead of a baseball, and in fact there are some compelling arguments that the baseball is the more logical choice. (Playing slo-pitch with a baseball, conversely, would turn many leagues, including my own, into a horrific carnival of laser-beam hits featuring thirty home runs and four cracked skulls per game. Actually maybe that would be cool.)
Even if you elected to have women’s baseball played with bases at 75 or 80 feet, instead of 90, it would still make more sense to the game to be played otherwise the same as baseball. And for the hugher levels of adult skill, of course women could play on a regulation infield. The throws are longer, but baseballs are easier to throw.
If you don’t believe me, try an experiment; go out and play catch with a friend, and bring a regulation softball and a regulation baseball. Spend 15 minutes playing catch with the softball, so you get lots of throws in after you’re warmed up and you know you’re throwing the softball as well as you can. Then switch to the baseball, and I swear to God you’ll feel like Nolan Ryan. The difference is amazing; you can fling the baseball effortlessly with incredible velocity. The whole point to a softball is they are harder to make go fast.
FWIW I don’t feel marginalized for having played 12 years of softball. It never even occurred to me that I was doing anything less or different than boys were. I loved being on a team, getting dirty, playing hard, saving up money for new gear, hitting home runs, being an all star, getting old and being stuck in right field, getting injuries, being traded, etc. I still looked up to pro baseball players and became a lifetime baseball fan.
My brother got hit with a ball when he was in his first year of kid-pitch (it starts out as coach-pitch) and that was it for him. I played like 10 years longer than he did, and got hit by the ball plenty of times. I’m grateful for my “slow” game - it never felt slow to me.
If it really was a resource issue, I’d think you’d have both boys and girls playing softball in middle school, then both boys and girls graduating to baseball in high school. Because in order to have both baseball and softball at the high school, you’ve got to have two fields rather than just one.
OK, the softball field is a little smaller than the baseball field, but still, the two fields will take up a good bit more room than just one.
I remembered that from high school. Two forwards (who could only play in the forward zone), two defense (who had to stay to defend their own net), and two rovers (who, unlike the others, could cross the halfcourt line. I loved watching it and found it more interesting from a strategy standpoint than the regular game.
They switched to the same setup as the boys a year or so after I graduated.