Pesticides are still widely used indoors. Pests will get introduced even with strong precautions (air showers at the entrances, etc.). At that point, if they're

Pesticides are still widely used indoors. Pests will get introduced even with strong precautions (air showers at the entrances, etc.). At that point, if they're able to multiply then they will, potentially even faster than outdoors since there are no natural predators. There also tends to be less airflow, which encourages pests like spider mites. Of course the greenhouse has advantages too--you can deliberately introduce beneficial (predatory) insects and they can't fly away, and if you do end up using pesticides then there's no risk to wild pollinators. For example, the EU banned certain neonics outdoors but still permits indoor use. It's more just different than strictly easier or harder.