Heart Rate not getting up like outdoor rides

I too have noticed its hard to get my HR up very high indoors. Reasons are numerous in my opinion, but limited competition and adrenaline are probably high on the list. So i have adjusted my indoor HR’s as roughly 10 bpm lower for indoor settings on my Polar GPS and work to this in conjunction with power and the efforts seems comparable with outside workouts.
As long as you know what your expected HR’s are , both indoor and outdoor, you’ll make progress.
I’ve also found virtual power to vary in different power zones with actual power, so again just record what levels you’re working at and benchmark training on them.