All cheap smartphones have a fingerprint sensor but all laptops dont have one. Why?

There are absolutely laptops with fingerprint sensors.

I'd say the main reason it's more common in phones than computers is because of the different markets. Phones are mostly consumer purchases, the business market is smaller and the software is more locked down so you can rely on a software disable better sufficing for those cases. Laptops are increasingly dominated by business use cases. Businesses have IT groups that care about security who would prefer models without biometrics.

Secondarily, you login to your phone a lot more often than laptops so the convenience factor is less impactful for laptops. So people don't consider the fingerprint sensor a mandatory requirement as much as with phones.