Bug queues grow faster than they get resolved because triage and fixing both require a developer's full attention. A developer has to understand the reproduction steps, find the relevant code, understand why it is broken, write a fix, and verify the fix does not introduce a regression. That sequence takes significant time even for a straightforward bug.
Many bugs in a queue are actually straightforward: a nil pointer that should have a guard, an off-by-one in a loop, a missing error return. They accumulate not because they are hard but because no one has gotten to them. Meanwhile, the bugs that require real judgment sit in the same queue and get the same delayed attention.