I'm guessing because capacitive fingerprint scanners are now considered old, outdated technology and the in-screen fingerprint scanners are new tech. So if Google are trying to sell high-end smartphones, might not look good to have old technology while other phones have all the modern scanners.
Not all in-screen fingerprint scanners are like the Pixels. Competitors have better tech scanners and even using the tech Google used, they still manage to have it read faster than what Google does. If you've used an S21 fingerprint scanner, you'll see how fast and reliable they can be.
Personally, I like in-screen scanners over the previous rear-mounted capacitive sensor. I have an office job so it's much more convenient for me to unlock the phone while the phone is on the table facing up, rather than having to lift it up and reach for the back.
That's not a good argument. If opening every app took a few hundred milliseconds extra to open, if swiping to the home screen took a few hundred milliseconds to go home when other phones in its class don't, no one would be justifying it by effectively saying 'you need to be more patient'. The fingerprint scanner shouldn't be getting a pass here either.
The point is we shouldn't have to press and hold for a 0.5 to 2 seconds. There are alternatives out there that are much faster and accurate.