There's some very good reasons:

• VPN traffic could be masking an attack on the account
• By using a VPN, they lose a degree of certainty that it's you, they can't use the IP address as a factor to establish the probability it's actually you
• Differentiating you as a person, from other people with the same source address, perhaps who are behaving poorly, or who've implemented robots to do things Google doesn't like.

I fully believe VPNs should be a fundamental right on the internet, nobody should have to identify themselves by IP address to use the internet. But from an account security perspective alone there's a good reason to be extra super duper sure of somebody before allowing them to log in