Why? Because of the heating elements and the timer. If the timer is "on" and the circ pump is "on" and you are running water around an uninsulated loop for 15minutes at a time, guess what else is "on"? THe elements! at what 1000w? 1500w? Thats like 25 light bulbs.
Tankless water heaters are not "Instant" water heaters! If you want "instant" you need some kind of recirc unit. THe ACT Metlund is the best thing going right now for residential application in my opinion for any hot water system and for the same reasons. Solve one problem without creating more.
BTW, if you have to delime every 6 months, you have other problems. you need a softener system. If you had a tank on that same rotten water, it would probably only last a few years, and if it were electric you'd have constant element issues. so nice try. At least you CAN delime a tankless.