There’s plenty of fw diseases - you just don’t hear about them as often. Columnaris, oodinium (freshwater velvet), and mycobacterium (tuberculosis) are just a few examples of diseases that can wipe out entire tanks if not caught early (and for many columnaris and myco treatments there is no effective cure).
First off, majority of common freshwater fish are captive bred, which reduces the amount that you see. Wild freshwater fish have many more issues to consider.
There’s also a cost factor - when you are paying an average of $5-15 per fish, you aren’t going to think about losses as much as if the fish cost 50-100, so disease gets more focus.
Treatment also tends to be simpler in fw - salt is an effective treatment for many diseases, and treatments can be done in the display tank due to a lack of any sensitive inverts. Marine tanks have more than just fish to be affected by medicine.