Why not just flash the ROM and install vanilla Android instead of settling for TouchWiz and other third party bloatware?

I don't consider any of Samsung first party apps to be bloatware especially since I use most of them on a daily basis.

While you could consider all the Samsung apps extra and unnecessary - the Android ecosystem is a loose group of OEM's with varying degree of control over their flavor of Android. The base features are always there on all android phones but Samsung personalized theirs. It's a good decision despite android purists scoffing at it.

Pixel line of phones is there to satisfy those who really want stock that badly they're willing to buy a flagship phone with budget phone features

Because there is no vanilla Android ROM that works perfectly out of the box on all devices and you need a device with a unlocked bootloader to even that get far.