Im not going to touch the technical difference in electronics between an Alembic and Fender, Modulus, etc....There are others here who can talk about that better than me. I do know what my ears and hands tell me however, and its that an Alembic is able to give me the sound I want, plus many others.
However, I also get a good sound or two out of my 64 and 72 jazz basses. They are both good sounding basses in their own right, and I dont have to add any eq or anything to get them to sound good. The key thing here is a sound or two....With my Alembics I can get many different sounds, most of which are usable. For me personally, since most of the bands Ive been in do alot of improv, this comes in handy.
Back to other basses, I have yet to walk into a music store and pick up a bass that was something I felt I had to have...whether it is modulus, sadowsky, etc..It could be that these stores were stocking the lower end of these models, but for whatever reason alot of these basses were lacking in something.
Economics are also a factor. I can afford to play Alembics. Im not rich by any stretch of the imagination, but so far Ive been lucky enough to get two.
Im not really sure how much money pros and more experienced players make, but given that I know a couple of guys that are pros (first call for studio work, first call for fill in situations, etc.) that really dont have alot of money (one guy like this I know is 45 and lives with his parents, but gets killer gigs), I suspect alot of these people play what they can afford for one thing, but you also have to consider that endorsements, higher production levels, and LOTS of advertising come into play. Just look in BP magazine...how many warwick, modulus, etc. ads do you see compared to Alembic ?
As to why some are so much more expensive, lots of reasons, but few are probably valid. Outside of Alembic, and maybe one or two other companies, I have not seen or played any instrument that I would feel good paying thousands for.
Also, alot of lower end bass models by big name companies look and feel lower end, even though your still paying big money for them.
I havent played every bass in Alembics line, but I have played a few, and even the lower end models show the same care, attention to detail, and craftsmanship that higher end models do. How many other companies can you say that about?
As far as ego having a factor in this (kind of baited question, but I'll bite), I can see how that can be percieved by others. When I show up at open mics with one of my Alembics I know people say shit, but who cares? Your here once on this planet and thats it. You may as well enjoy yourself...