Many of them have been remade. The first two Monkey Island games are now in “special edition” format on Steam and GOG.com. Some of the early Sierra ones have been redone, there’s a new Leisure Suit Larry remake which is decent (and has a couple of extra puzzles over the original) and the first three Kings Quest games have had overhauls and have the added bonus of being free.
The original Gabriel Knight has a 20th Anniversary remake (in a nice bit of synchronicity, I started playing the original version this morning as it’s one of the few that passed me by originally). GOG recently acquired the rights to all the Lucasarts games, so things like Sam & Max are available from there with more to follow.
Speaking of Sam & Max, Telltale Games have done three modern series of their episodic games featuring the erstwhile crime-fighting duo, and whilst the Lucasarts original is still the best IMHO they are very good. I’m halfway through the last series now and really enjoying them.
The Broken Sword series are available for tablets (I picked them up in a Humble Bundle offering a bit back).
Tim Shafer (of Grim Fandango fame) Kickstartered a “Double Fine Adventure” which became Broken Age. I’ve not played it, but it looks like it could be fantastic.