Even ignoring the impedance mismatch, loss in typical 75 ohm coax at 2.4GHz is going to be something like 0.5dB per metre. Far worse again at 5GHz. That's going to add up quickly.
If nothing else is on the same run, the amount of noise from external sources should be very low, which in many noisy RF environments is fine when you are only going 10-20 meters through a building. The transmit power of a wifi card is going to be way above the amount of losses that would cause issues.
The loss from air itself is a tiny fraction of a decibel per kilometer.
But using an antenna means you lose 40-48dB in the first meter for 2.4-5GHz. And then you lose 6dB for each doubling of distance, so another 12-18dB for 4-8 meters.
If you're less than 20 meters from an antenna, not only do you have those massive upfront losses, you still lose more than .5dB for each additional meter of distance.