I feel like it’s not so much creators are “hesitant to attempt new villains”… as it’s been pointed out, there’s been many… they just haven’t caught on and endured long term. But I think there’s just more world building in the Batman comics in general. Maybe there’s more collaboration between different creators or just a greater sense of forward momentum with the books. I also don’t think this is a problem for other Marvel books either… the X-Men, Daredevil, Iron-Man, etc have all had substantial additions to their rogues since 2000 compared to poor Spidey.
Heck, we can even rewind the clock further to 1990 and there’s not that much more to show. The super popular Carnage, of course, and maybe Kaine even though he’s no longer a bad guy… and then a couple C-listers who appear very seldom like Cardiac, Lady Ock, Shriek, etc.