Hee~! All the recent topics in this forum seem to be concerned on issues of the status of AG. Why have the question of
"why can't AG of today be like that of the good olde days?" arise all the sudden?
Anyway, to answer to your query of why today games can't be as good? It's all down to budgetting issue. In the good olde days, there are money for developers to design whatever games that could capture the crowd, since there were actually a big group of people (proportion-wise, of course) willing to pay for the best games AG around then. This in turn invited competitions, with most of the games in the market being either action or adventure at that time, many companies started to release clones (Myst clones or XXX Quests clones), while some more ambitious developers stood by and created original ideas which would ultimately stand the test of time, and went to become a legendary status. Many of the LucasArts games were such legends, leading the pack in how stories were told (Grim Fandango, Secret of Monkey Island, Day of the Tentacle, Sam & Max: Hit the Road etc.), and created outstanding puzzles that were much more sophisticated than the key stuck in the keyhole of the other side of the door.
Nowadays, with lack of a large base of potential consumers, fewer publishers are willing to stick their neck out for untested ideas. Hence most games keep to the same-olde tested formulae, making the scene look a bit stale. Still, once in a while some great games to come to pass. Just look out for Indigo Prophecy and DreamFall!