Lots of people. It’s great that you don’t, really, but Calibre is very ugly¹ and I have no doubt that limits its adoption. Subconsciously, people associate ugly apps with unpolished or unfinished apps.

I don’t use Calibre (I rely instead on command-line and bespoke tools) but I do recommend it to less technical people and I always make it a point of saying “it’s very ugly but it does everything and does it correctly, it’s the gold standard for what you want to do” because I’m pretty sure if I didn’t add that disclaimer the people I recommend it to would think they downloaded the wrong thing or abandon it as soon they opened it.

¹ That’s just personal opinion, we don’t need to have a debate about that. If you find Calibre beautiful, more power to you.