Lots of reasons.
1 Bad light.
1 Sure, you can see the spinner easily enough, try picking up a Jofra Archer bouncer in bad light. Dangerous. That is why the old rule used to dictate that the bowlers mattered for light. If the fielding captain wanted off the field, he'd bring on his fastest bowler, if he wanted to stay on the field, he would use spinners and medium trundlers. Batsmen could be offered light until deemed dangerous.
Wet pitch. This is unfair for the batsman, and turns the pitch into a sticky dog, so its best to get the covers on. A sticky dog will seam, spin, and variably bounce. Noone knows, not even the bowler, what any delivery will do. Its just so variable. So a dry ball and a wet pitch, bowlers dominate. Remember Bradman famously reversing the batting order? That's why.
There is a flip side, though. A soapy wet ball is more difficult for a spinner to grip. But even as a seamer, you try bowling slower balls and control a wet ball. Also - any chance of reverse swing just disappeared altogether as soon as the ball gets wet. This is commonly seen in the IPL and India in evening games where massive dew forms on the outfield. It makes the ball completely wet. That is why teams prefer to bat last and chase in India at night where dew is forecast. Its so difficult to bowl at night when there is dew.
No these two do not "even out". It depends on the shower and the drainage, and more importantly, when the next new ball is... or when the fielding team can change the ball to a dry older one cos its "mis-shapen"...
Cricket is like no other sport I know in that the pitch and the forecast is so so so so determintive of the result. Sure in football you can have long grass or short grass, rugby you can have big in zones or small in zones, but nothing like spin vs seam. And if rain is forecast, you chase. Always chase.