And many other frameworks did it too. LÖVE decided it's out of scope, and that's a 100% valid choice, considering the breadth and complexity of the problem. They still went quite far in ensuring it's no more difficult than it actually needs to be. Similar with e.g. cross-compiling Go programs (unfortunately, no cgo-less, cross-platform framework for interacting with windowing systems/video hardware exists).
Yes, but most other engines are much larger scope and fundamentally different purpose than LÖVE. DragonRuby, however, is in the same camp: code-first, scripting language (Ruby vs Lua), beginner-to-expert friendly. So my point was there are (at least one) project(s) very similar in purpose and size/scope to LÖVE that did solve for cross platform.