In Safari websites have to ask permission before it can send you notifications. They can't just 'spam' you. You can also completely disable them so you're never asked for permission. I don't use Chrome so I don't know what the norm is there.
Since people visit tens of new websites everyday, asking permission once is also close to spamming -- it's a non solicited popup for something most don't care and don't want.
And don't get me started with the BS "This site uses cookies, OK/Not OK" we get in Europe...
Source? From my experience people tend to visit the same few sites everyday. Personally I've seen that permission notification about 4 or 5 times in the several years it's been a feature.
It's a power distribution. They visit their few sites everyday, but they also visit a long tail of sites for different things.
Except if we're talking about Aunt May/Uncle Paul, who only go from their Outlook Mail to Facebook.