xml != html so you can't just plug an xml parser and a schema together and call it quits.
Plus, there's things like browser support, relative paths, and content embedding to consider, which always made dedicated products like Adobe's (Macromedia's?) DreamWeaver hit-or-miss (on top of being a very clunky way to edit some tiny text files).
As for why it took so long...well nobody cared enough, and everyone "just used scripts" i.e. Django, Wordpress, Ruby on Rails, etc, to get that "good enough" experience + access to testing. There's no reason this couldn't have been done back in 2014 when LSPs were first just starting to come into being...
Plus, there's things like browser support, relative paths, and content embedding to consider, which always made dedicated products like Adobe's (Macromedia's?) DreamWeaver hit-or-miss (on top of being a very clunky way to edit some tiny text files).
As for why it took so long...well nobody cared enough, and everyone "just used scripts" i.e. Django, Wordpress, Ruby on Rails, etc, to get that "good enough" experience + access to testing. There's no reason this couldn't have been done back in 2014 when LSPs were first just starting to come into being...