Usually it's just a factor of the thin-ness of a laptop. Ethernet is not a small port and laptops that aren't heavy duty ultrabooks are regularly thinner than one, even at their thickest point.

You can get USB->Ethernet adapters.

Exactly. They're very cheap, anywhere beteween $5 and $15. Cheap enough to basically add them to every ethernet drop where you regularly sit.

For simple things, I actually love my HP Stream 11 laptop. Basically a ChromeBook sold with Windows. No Ethernet.

My kids' ChromeBooks don't have an Ethernet port. Good little Acer machines. School-issued CB, I have not been allowed to examine yet.