The toilet seat is typically cleaner (in terms of bacteria) than the office desk. Certainly aerosol contamination is a risk, but that can easily be solved with a protocol of leaving the lid closed. And that's all assuming that bacteria are outright bad; it's becoming more apparent that too little exposure to bacteria is dangerous as well (can lead to auto immune disorders). Obviously I'm not suggesting living in faeces, but a bit of pragmatism can make something like this not a problem at all.
And it's easy for a kitchen without a toilet attached to become disgustingly unsanitary.
Especially not if it's your own poo. You might keep reinfecting yourself with some bacteria or parasites that would otherwise have a slim chance of staying in the body, but the human immune system is pretty well-equipped to handle that.
Having your own toilet in your own kitchen is probably more sanitary than separating toilet and kitchen, but sharing them among multiple people.
And it's easy for a kitchen without a toilet attached to become disgustingly unsanitary.