iPads are cheaper than MacBooks and more popular. They'd rather prefer if you bought another one instead of using it indefinitely. The same with smartphones. The answer always has been: I like money!
iPads and MacBooks are architecturally different devices with different purposes (but soon the difference may vanish). People tend to upgrade their phones/tablets more often than their PCs/laptops. Macs aren't locked down because they are designed NOT to be locked down. You can write drivers for macOS (otherwise it couldn't compete with Windows or Linux) but not for iPadOS.
