Sounds like a case of bad RNG. It's not always that bad. The devs didn't design it to do that on purpose, but random spawns are something to think about when prepping a boss site. It will get worse in the plains if you don't do any prep work.

For wolves and golems a trench or earth wall will keep them out. Make sure to keep the battle area big enough or moder will go land somewhere else. That won't help with drakes but if you know where they're coming from, a few well placed campfires will prevent respawn, like if there's a nest nearby.

Spamming campfires around any boss area, when you can, is a good idea. The anti-spawn effects still work and they're less likely to be targeted than workbenches.