BIAB IS all grain brewing. But it's not traditional multi-vessel brewing. Some batch sparge with a cooler, others fly sparge with a sprinkler, some use a bag for the whole water amount (BIAB). All-grain brewing, each of them.
As for sparging, do note that some BIAB brewers add a sparge step, where they may pour a gallon or so of hot water over the grain bag while it drips over the pot. It does add efficiency points for most who do this. So not all BIAB is sparge-less. Lots of ingenious methods to make wort. Also, large grain bills for high-gravity beers can be unwieldy and might be difficult to manage, or fit as a one-bag brew. In that case, a multi-vessel system can be most beneficial.
As for partial mashes, if you have a recipe that's a partial mash you might not want to spend the time, energy, and money to find the exact all-gain solution. Also, lifting a wet, full grain bill out of the water is heavy and can be messy. A pound or two of steeping or partial mash is much easier to deal with.
Many BIAB brewers find their efficiency in the 80's, higher than traditional methods. Crush it mightily!