Firstly, they tend to be import-only so obtaining them from Japan via places like ebay is usually the only option. The other issue is that they can be expensive! Naturally, soundtrack CD's are a little specialist so they aren't often produced in huge quantities like general, commercial CD's which quickly become worthless and are printed numerous times, always being available from somewhere. You also have to consider that game music (especially for older stuff) isn't readily available from the likes of itunes or other digital music vendors so unless you are going to download the music or torrent it from somewhere, an imported CD is sometimes the only legit method of treating your ears to some great tunes.
I personally think that it's all a matter of perception with people seeing a CD that costs more than £10 and believing that it isn't worth it just because it is a CD and CD's rarely retail for more than £10 brand-new in the charts. The point that they are missing is that soundtrack CD's aren't mass-produced and actually have collectable value to them. It's the same for a lot of movie soundtracks too. For all intents and purposes, they are a completely different product.
I don't have a big collection of videogame soundtracks myself but I do really appreciate them and am always after more as and when funds permit. Here is what I have so far...
Tekken 2 Strike Fighting Vol.1 + Vol.2
These two CD's contain all of the arranged music from Tekken 2, one of my all-time favourite games. The arranged OST for this game was simply stunning with each theme being distinct, atmospheric and memorable. This was Namco's sound team on a real high and it sadly went downhill from here. Tekken 3 was decent and while the following sequels all had a few standout tracks, the OST's as overall products never impressed me quite as much.