Because there is not central cushion the dished seat deforms your hips. You think the chair is comfortable at first but after about 4 hours of sitting at the office it really starts to wear on you. Adding a cushion helped a ton but you shouldn't need one for a decent chair.
I needed to replace my previous chair of almost 15 years and wanted one that was completely mesh on the bottom and back and was also comfortable for long term sitting. I’m happy with this one so far after a couple weeks. Could be a little more wider and deeper in the seating area but it works great.