Except the way most people do backups, by the time they are made aware of the ransom, their backup files are already overwritten with the encrypted ones.

I would also add, multiple backups in multiple places.

Just for my Windows PC I have three backups. One on the machine, one on an external hard drive and one more in the cloud.

How much did it cost me? $80 for the 2TB hard drive and $8 a month for unlimited cloud storage and backup. A small amount to pay to make sure you're covered in nearly any disaster scenario, including ransomware.

The Mac makes this so easy with Time Machine. Is there something that's equally easy and effective in Windows?

Unfortunately, not really. Backup on Windows is strangely terrible out of the box. The best solution for Windows is probably Arq.

Why couldn't the ransomware encrypt your Time Machine drive as well?

It certainly could do this. I don't think Time Machine protects its data in any way.

Backups have to be coupled with some kind of way of noticing that something is wrong. If the ransomware encrypted your data slowly over the course of months and you didn't notice you might be out of luck regardless of your backup system.

Don't keep the drive always connected and you're fine