Combat is always situational. Make their bad rolls have environmental consequences.

From a narrative standpoint, a bad roll doesn’t have to be a miss. They can hit a barrel of oil, or fish, and cause it to break open. Now everyone who isn’t a fishmonger within the spill area has disadvantage on all attack and save rolls.

Also, as dm, you never have to make your roles public.

Give them custom weapons or spells that are most effective when you roll low.

The luck system might be something to look into. It can help mitigate consecutive failures while maintaining balance.

You might consider adding a puzzle element to encounters that can lower the effective difficulty with clever maneuvers or strategies.

As an example, fighting in a room with a big chandelier overhead. A player can cut the chandelier at the opportune moment to pin a major adversary, allowing them to coup de grace or simply flaunt their victory to the villain’s face.

Or perhaps fighting in a room full of mirrors that allow a clever player to reflect a gaze attack. Or doing Battleship style combat, where you have to pick the square of a hidden enemy, but you guarantee a hit if you guess correctly.

In general, try introducing non-dice resolutions to the scene - guessing a magic word that disables a key piece of enemy tech, baiting enemies into an area or formation before springing a trap, completing a ritual that can summon a powerful ally by solving a rubric cube.

If all else fails, you can drop some nice loot them. Awand of fireballs or Staff of the Sun or similar high powered magic item, for instance. Doesn’t matter how you roll with these, you’re going to have some fun.

After I noticed this, to confirm it wasn’t just imagination I just started logging the roll results (d20s, at least) into an Excel sheet as we played. And yeah, they’re actually rolling that badly.