So much to do, and unfortunately, those technical tasks often simply don't make it into the sprint. Over time, not having that technical keep up causes increases in technical debt and slows down delivery. Eventually, you have a lot of technical tasks, and yet everyone else still wants new features. Velocity is slowing down, builds are taking longer, and there's more general muttering about things not being clean in the development and test environments.
It's time to introduce the sprint renegade.
The sprint renegade is someone who effectively leaves the team for a sprint. He goes off and simply works on tech tasks to make it easier for the rest of the team to meet the obligations of the backlog. This reduces your potential velocity for that sprint by one person, but it boosts your long-term velocity by taking care of some of the drag on the team. Repeat this for each sprint until velocity is back up where you want it to be.
The general idea of a sprint renegade is that internal tech tasks are very hard to sell onto a backlog, and they will often lose in the face of customer-facing tasks. If that's true in your company, sometimes drastic measures are in order, temporarily.
If your technical debt has crept up to a point where it's interfering with delivering, find yourself a renegade. You'll be glad you did.