Because in a lot of software engineering jobs we are given a lot of trust in managing our own workload and this is a breach of that trust.
If you have too much on your plate you can (in well functioning teams) tell your manager "hey, we can't do X" or "we need to push the deadline on Y" and that will be taken at face value. You won't be expected to put in 80 hour weeks just to get it done outside of extraordinary circumstances.
The flip side of that is that if you have so little work that you have time to work a whole other job you're meant to tell your manager "hey, I have spare cycles, what do we need to get done?".