Can you buy licenses?

I can't, but OEMs can.

Why haven’t all manufacturers making and selling ARM tablets / laptops with Windows ARM then?

Because no one was buying them.

Microsoft was pushing to make this happen extra hard with Windows 8 or so. They've kept it alive since then. It's revived a bit once they started seeing ChromeOS devices start taking over the low end of the market.

But the available ARM processors kinda sucked; the price difference at the low end wasn't enough for consumers in most cases: low margins are a demotivator for manufacturers; Intel and AMD got better low-power, low-price options; app availability was/is a big problem, etc.

But you can buy Windows ARM laptops and things from Samsung, Lenovo, a few others right now. The others don't care, mostly still for the reasons above.