Long term side-effects may develop over time and may therefore not be caught in these studies. Longitudinal studies are needed to study this. As an anecdote, a relative of mine has has had dbs for years due to brain disease and has developed paranoia. May or may not be related.
My sibling has DBS for turrets. Turns out a side affect they warn you about is that you might forget how to swim. The neurologists strongly recommended caution when in the water.
Because you're only hearing about the ones that worked out. Unless you're deep in the weeds you aren't hearing about the things that are being tried but don't work.