You actually do.
Spiders tend to crawl into your mouth at night because they prefer warm and moist places. This is a known fact.
This is a known urban myth.
Spiders would be deterred from a hot humid breathing mouth expelling carbon dioxide at them.
You would also feel a spider entering your mouth and your body would naturally defend itself and your reflexes would close your mouth when it touches the lips or brush it away, or wake you up. You'd have to be a heavy sleeper to not notice anything like that entering your mouth.
Spiders would have no fascination of being in a human mouth because that sort of location is not where they find their prey, namely house insects which normally prefer dryer locations, such as lights.
There's certainly no way that spiders regularly enter into mouths and become eaten.
And there's no credibility to that old myth that people will on average swallow 8 spiders in their lifetime.
(By the way, if that myth were true, you'd swallow 8 spiders across an average lifetime of 80 years (avg 79 US, 80 UK), meaning that you are likely to swallow a spider once every ten years. That's not common at all.)
