1) Why are Stalkers paid large sums to bring back alien artifacts from the highly dangerous Visitation Zones? Becuase building J-Drives, and possibly M-Drives as well, requires such artifacts. Furthermore, research into better drive types and assorted gravitic artifacts requires such artifacts.
2) Why are J-Drives as unreliable and finicky as in CT-LBB2? Because they use poorly understood alien artifacts at their cores. These artifacts have negative interactions with strong planetary gravitic fields, and are very sensitive to things such as unrefined fuel, leading to misjump.
Why not both? I think it'd make a fairly interesting universe if the aliens are the strict definition of "higher TL":
Humanity has J-Drives, but they're more finicky, less reliable, and much more massive. However, they can be rebuilt and repaired.
Alien artifact J-Drives are a mixed bag: They're definitely more compact and more reliable. They may
have higher Jump numbers, actually work, and be able to move more mass. There's no way to know. However, Stalkers bring back J-Drives and everyone bids on them anyway because certain military/governmental/corporate applications use alien J-Drives - scout ships, fast couriers, and large starships might all depend on alien J-Drives. Alien J-Drives are best repaired by alien J-drive parts -- humans understand enough of alien J-Drive technology to break them down into parts and reassemble them - it's like in the modern world, someone might live in a country that can't build all the tiny microelectronics in a cellphone, but that doesn't mean that some technician with a soldering iron and tweezers can't do quite a bit of work on a cellphone provided the technican has access to a stock of replacement parts. Similarly, humans understand alien J-Drive parts, but it's just beyond our manufacturing capability to make them.
Humanity does has useful gravitic technology but it's very limited. By careful reverse-engineering of alien ones, humans have come to understand gravitic technology, but it's not that useful yet - it's power-hungry, bulky, and the fields are not very strong (eg; we haven't reached break-even yet - we can't make a gravitic manipulator that can cancel its own weight). Earth-tech artificial gravity is also a bit too massive to be practical to provide artificial gravity in all but the largest starships, though it's seeing use in space stations or to augment local gravity on places like the moon.
Alien grav tech is in stupidly high demand, therefore. Anyone who wants artificial gravity in your "average" spaceship that isn't provided by spin capsules (or whatever) has to do it with recovered alien technology. It's an incredible luxury - super-executives of huge corporations, high-ranking national leaders, and grossly powerful/wealthy types are the only people who can afford starships with full artificial gravity (of course if you want to rub in the "universe of inequality" message, anyone who can afford a ship with full alien grav is able to afford multiple
ships with full grav). Many long-haul ships where crews are expected to spend the better part of a year in space have "gravity decks" in an effort to save money - like the majority of the ship is null-g, but perhaps doctors have discovered that 8 hours a day in 1G is sufficient to hold off the bad effects of low-g, so the bunk rooms use artificial gravity while the rest of the ship doesn't - such a gravity deck would be as small as possible and hot-bunking would be standard even some 10-person crew ship. Similarly, like the decorated trucks of India and so on, it's a sign of wealth and prestige for a free trader to have a small alien artificial gravity plate right under his captain's chair - a captain might spend 16 hours a day in that chair so it's best there - some free traders might have similar plates for the entire bridge crew, bought after a particularly lucrative period (though more common are the tales of "ha ha, yeah back in '58 I used to have an entire gravity deck for the bridge, but then hard times came and I sold it all off").
Stalkers are paid stupid high amounts for the tech they bring back because it's the only source for these high-end items, whose value has been artificially inflated far beyond their actual use because of their status. It doesn't saturate the market because even if it is relatively common in alien sites, the items are hoarded - in this world, alien magitech still breaks down and still requires maintenance. So if you want to have that gosh-wow alien grav plate in your ship, you also need to budget to have a stock of very expensive spare parts on hand. These are all cribbed from existing ancient alien items, so there's no guarantee that the replacement parts will work for very long, either. It also means that even non-working items will sell nearly as well as working items; the sale value is by how intact the devices are, even if they don't work; those who buy them know that an intact item, even if it doesn't work, will still yield a good profit because spare parts are so in-demand.
Between human drives being more primitive and the alien ones being theoretically higher reliability but in reality they've been rebuilt many times and all of them are literally thousands of years old (though not constantly used the entire time), I think explains why Jump Drives are so unreliable.