Perhaps because Apple doesn't release what customers want?
And maybe, just maybe, because Apple would otherwise decimate MacBook sales?
It's not that difficult:
Apple is a profit-oriented company. They are therefore very careful to develop each product in such a way that customers are compelled to buy all of their products.
Of course, they talk about “pro apps” for the iPad. Marketing 101.
But of course, that's all nonsense.
Because, as I said, if the iPad (Pro) were good enough to replace the Mac, Apple would no longer sell Macs. So, in order to force customers to buy both, they have to withhold features from the iPad. Which is what they do.