With adventure games, some people are just very persistent and can guess what the designer was thinking. While the apparent search space can seem huge, trial and error(use all verbs including the look-at; use all item combos), reordering actions, and pixel hunts eventually defeat everything except the puzzles where search space has been deliberately inflated(secret code puzzles, infinite mazes etc).
The Lucasarts games were not the hardest in the genre by any means. By the time they came along there was a huge legacy from both Infocom and Sierra and those also had parser issues(you wouldn't be able to trial-and-error verbs or nouns without sitting down with a dictionary) and softlocking events(don't eat that custard pie, King Graham, you might need it later).
It's interesting to compare with Japanese adventures, which gravitated towards a straightforward menu of choice actions by the late 80's but mostly stayed in first person instead of using an onscreen avatar, and from there, visual novels became a standard gameplay style. (It takes a lot less animation to show first person scenes, and many of the titles were targeting the higher-res display tech of PC-88/PC-98, which might have something to do with it) The Ace Attorney games are good examples of this style of game, and they have a particular strategy of focusing on one style of interaction at a time(separate scenes for investigation and trial).