There's more to cases than pretty lights, it's all about airflow and passive cooling. Unfortunately the market at the minute prioritises those things so the best cases tend to also have loads of blinking flashy lightsSame scenario for me. Besides which, unless you attend LAN parties (whatever one of those is) what's the point of all the RGB stuff?
OK, your new computer arrives. You take it out of the box, plug everything in and press the GO button. It all lights up with all the pretty colours, flashing lights, ringing bells, etc. You go "WOW, that looks AMAZING.😁😁😁" 6 months down the road, you'll barely even notice.
Yes it's "the fashion" , or "it's what buyers want" , lots of other 'reason's' for it.