OK. Well there are reasons. They may or may not apply to you. In no particular order:
1. Realizing losses introduces the potential for wash sales, which should be properly understood and managed.
2. If you realize losses that exceed your gains plus $3K, you'll suffer the economic loss
now but will not receive the tax benefit until
later, or
not at all if you die with a carry forward loss (unlikely you're that old).
3. The realized gains might be taxed at 0% or a low rate, so may be no strong reason to erase them.
4. If you like what you own and it's temporarily down, the wash sale rule (see #1) could prevent you from participating in the recovery if it happens to go up a lot in the 30 days after you realize the capital loss.
5. The STCL might be more valuable offsetting STCG instead. (Sorta related to #3).
(I'm not particularly fantastic or sophisticated about this sort of strategy stuff, but the above would be things I would think about. Paging
@MDM.)