Because it's you. You're using the same Google account that was used on the phone before. If you try any other account on it, it will ask you to enter the one used prior to a factory reset. And you're entering that. FRP doesn't protect against the owner. You're the freaking owner. It protects against use by anyone other than the owner. Also this has just started on Lollipop or Marshmallow depending on the brand on device you used. So if you previously had an older device, it may not have had it.
https://m.androidcentral.com/factory-reset-protection-what-you-need-know
Even Google's and Samsung's own support website says you should remove your account properly before factory reset.
https://support.google.com/nexus/answer/6172890?hl=en
https://forums.androidcentral.com/e...ng.com%2Fus%2Fsupport%2Ffrp%2F&token=M5xKpQeZ
I sell all my phones after factory reset. That's the only time I do a factory reset and I never log out of Google or Samsung. But I've never heard of anyone complain that the phone is useless. (I have sold about 6 phones so far)