Because it's almost always rubber solution rather than glue, so a patched tube is almost as good as new - just slightly thicker in that spot.
Whereas some ready-glued patches use actual glue instead and so you can peel the patch off, or it'll peel itself off inside the tyre if you're unlucky.
So glued is usually more reliable if you've time unless you've tested your ready-glued ones.
Except for small thing, in over 20 years using glueless patches I've only had three fail, my very first one because I didn't prep the tube right, then I tried a cheap Skab brand of patches which I was stupid enough to try another when the first one failed.
Once I tried to peel a glueless patch off a 5 year tube and the patch tore the tube with pieces of the tube attached to the patch...not really something you can just peel off.
I even once due to a forum discussion that glueless patches won't hold to high pressures, which was my argument that they could because I use them on road bikes, but regardless I decided to test a glueless patch to the ultimate test. I put a hole in spare tube that had an old glueless patch on it, and then patched it, installed it on a tire and rim and stuck the rim in a large trash can with an air hose connected to it with a remote trigger all of which connected to my compressor, I proceeded in putting in 240 psi, yeah me too, I was surprise the tire didn't blow off the rim or the rim didn't break, I kept that pressure on for a week, the patch never failed, I then let the air the air completely and put it back in my seat bag to be used as a spare, I had that tube and used it for 5 years after that. I once had as many as 13 glueless patches on one tube and none ever failed. I did find out that glueless patches don't work with latex tubes, they'll hold for about 2 to 3 days.
The only glue on patch I've used (besides the Skab tube patch) that fails by coming undone are tire boots, they last about a week, but at least you can get home with them so I always carry one.
Like I said before and I stand by what I said, I had just as much success with glue on as I've had with glueless. I'll never go back to glue on. I only buy either the Park brand or the Specialized Fatboy ones which I don't think are made anymore, or the 3M brand which invented the glueless patch and they're not made anymore either but sold the rights to Park and may have also sold the rights to Specialized but I can't confirm that. I can't bring myself to try other brands because I don't want to find that the patch doesn't work.
Due to another forum debate someone mentioned they used the black Gorilla tape, so I tried it, it held for about a day.