Because they're rather expensive, and that's a lot of MCr to tie up in machinery just sitting in a warehouse when you could be investing it in something that generates revenue.
If your business is providing spares to a hungry shipping maintenance industry, having drives available is your mechanism for providing revenue.
Depending on the time of day, it seems that the Traveller Universe is either teeming with starships flying hither and yon, or the ships are as rare as hens teeth with folks stuck in backwater systems, out of contact for ages.
On earth, as ships get larger and larger, their requirements get more and more specialized. But part of the is simply because large ships are rare and each one is pretty much a one off (barring perhaps 1 or 2 sister ships). And, I'm not speaking to the military, just the civilian market.
But as the volume of sale increases, then everything else scales up. If Z drives are super duper extra rare, then parts and spares and expertise for those drives will be super duper extra rare, even to the point that someone may well choose to not use those drives BECAUSE they are so rare.
If not, if Z drives are simply "uncommon", there will be a market to support those ships and those drives. Nobody likes downtime, downtime is expensive, and it's worth paying money to avoid it. Which means it's worth someones time to assemble a Z drive and stuff it in a warehouse somewhere knowing that soon enough someone will come wanting that drive and is willing to pay for the parts it's made of, the labor to assemble it, the warehousing fees where it's been stored all this time, and a little on top for the proprietors trouble.
Will every system have Z drives just sitting idle? Not necessarily. But, as the famous quote says: "Geographical oddity. Two weeks from everywhere!" In most sectors, that's pretty true. At worst you're likely 4 weeks from most anything you want.
Because when it gets to be more than 4 weeks, folks start tooling up to make it locally so that it doesn't take more than 4 weeks to get the item. Locality has value.
If a Z drive can be built is less than 4 weeks, well, there's no real reason to call out to have it delivered, is there?
Each locale will be a little different. Z drives may well be quite rare on the frontier. Rare in use, rare in support. Kind of a self fulfilling prophecy. Nobody uses Z drives because nobody uses Z drives. No one is using them, so no need for a support network. Since there's no support network, no one is using them.