I just played a board involving Minion and Lighthouse. I pretended there was nothing but those two cards and Province and just robotically purchased the highest one I could each turn. My opponent, who was of the ridiculous opinion that other cards matter, got crushed. It was the least fun Dominion game I ever won.
Minion by itself is just that good. I've won games buying nothing but Minion and Province. (Though not against people who are experienced, because in that case if it's a board like that they'd probably contest me for minions, and 5 minions isn't enough so other cards become more important.)
Minion works especially well with non-terminals that give you coin. Festival, market, peddler, fishing village, and yes, lighthouse - all those would work *great* paired with minion.
I think you're seeing the amazingness of minion rather than the amazingness of lighthouse. Lighthouse is quite good, but it only seems broken like that when it's in combination with either minion or library (or watchtower).
I once had a game where I think I had bought nothing costing 5 or more besides 5 colonies. (I played a fishing village/lighthouse/watchtower engine, set up by I-don't-remember-what (ironworks or talismans?) ).
So yes, maybe on that board you played, lighthouse-minion was the dominant strategy, and you saw that and your opponent didn't. So what? Many boards have a dominant strategy. BM-smithy or BM-envoy dominate things when there's nothing better around. Minion is one of those cards that can make a strategy on its own, especially when it has a good support card around.