Because you can do things with XSLT that you can't do with pure HTML
For instance, I can provide an XML file that is a list of widgets, and then I can use to XSLT to give you a table with the values sorted in some way. I can't do that with plain HTML, I'd have to invoke Javascript or something.
Okay, let's say I have a list of widgets that's in an HTML table. How do I sort the list if I don't want to use Javascript or install some programming language on the web server I'm using (let's say I'm using shared hosting and don't have admin access)? I could sort it by hand, but then I'd need to re-do the whole list if I decided to sort it by another column for some reason. And then if I wanted to add entries, I'd have to insert them by hand into their appropriate place and hope that I don't make a mistake
For instance, I can provide an XML file that is a list of widgets, and then I can use to XSLT to give you a table with the values sorted in some way. I can't do that with plain HTML, I'd have to invoke Javascript or something.