What is the reasoning behind not separating Sch and Smn stats?

[Alhanelem]

Because they want you to choose the one you'd prefer to strengthen most. Anyone who calls it a lockout is insane, you can still be an effective SCH if you statted for SMN.

You can also buy stat resets if you demand that you be able to be the best at everything.

The game isn't alt-class friendly anyway,

This is a load of crap as well. This game is alt class friendlier than any other game out there. There's no rule that says "to change to this job and play it for any purpose, you must have all ilvl90 gear."

I'm asking why? And pleading to separate the stats of Summoner and Scholar from each other so we can freely switch between them at any time and play either role at their best

No. ACN gives you a 2 for 1 deal on jobs. They want you to choose one of them that you'd like to favor. It does not render the other job unplayable, no matter how much you'd like to think so. This is another case of "wah wah, I must have everything let me ahve everything! gimme more, more is always better!"

This simply occurs because most humans are unable to make decisions.