Don't get me wrong, SLAAC also works fine, but is it solving anything important enough to justify sacrificing 64 entire address bits for?
* deriving additional addresses for specific functions is great (e.g. XLAT464/CLAT)
* you don't get collisions when you lose your DHCP lease database
* as Brian says, DHCP wasn't quite there yet when IPv6 was designed
* ability to proactively change things by sending different RAs (e.g. router or prefix failover, though these don't work as well as one would hope)
* ability to encode mnemonic information into those 64 bits (when configuring addresses statically)
* optimization for the routing layers in assuming prefixes mostly won't be longer than /64
… and probably 20 others that don't come to mind immediately. I didn't even spend seconds thinking about the ones I listed here.
"I wish to participate in a global telecommunications network and I wish to connect immediately to all my friends and be available to them 24/7 and I wish to play games with strangers across the country and I wish to receive all my email within 300ms with no spam and I wish to watch the latest news from Iran in 4K streaming Dolby"... but priiiiivacy!
Also we can always reduce the standard subnet size in 4000::/3 if we ever somehow run out of space in 2000::/3 (and if we don't then we didn't sacrifice anything to use /64s).
With SLAAC, it's just another implementation detail of the protocol that you usually don't have to even think about, because it just works. That is a clear benefit to me.
Plug in a rough router and see quickly you can find it.