Why is there not option to export assets as .JPG for developers?

I completely agree. Please go upvote this feature request to tell Adobe that you'd also like to see this feature added Download assets as JPGs from Design Specs – Adobe XD Feedback : Feature Requests & Bugs 

Currently XD is more focused on design than asset production (which is a bit of an after thought). I think Adobe is assuming that a lot of image production will be done with Photoshop (or a CMS - content management system where people upload PNGs and let the system convert to JPEG). For some workflows that's correct, but not all. I'd like to see XD get better at producing final graphics (from the app and from the share link feature).