Because you very rarely need to actually log in on your laptop. You lock and open your phone dozens of times per day, but you’ll probably log in once or twice on your laptop and that’s it. It’s not a feature many people would care about.

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.

What about the fact that fingerprints make great usernames but trash passwords? Perhaps the poor security and extra hardware and software are enough to discourage makers, they can add a variant with a FPS and if that doesn’t sell at all they won’t make many others.

The following is just my own assumptions, take with a grain of salt.

Phones are often taken out to perform a quick task, then stashed back in a pocket/bag many times a day. As such, being able to unlock a phone quickly is a rather useful feature.

On the other hand, laptops are usually taken out to be used for larger tasks, and as such, do not need to be unlocked as often.

Fingerprint scanners add less value to a laptop than they do a phone.