That’s true of null as well, depending on the type of site tho. I mean if I’m running a DED site in null and a red enters I wont immediately warp out but rather watch dscan for scanner probes. If they don’t have combat probes there’s no need to react at all.
Anomalies, on the other hand, are immediately accessible and it makes no sense to stick around in a PVE ship waiting to get dunked. Dscan is no defense there either, since we have cloaky + dscan-immune tackle ships now. If a red enters you basically have to assume they can appear right next to you at any moment, pin you down and cyno every ship in the game right on top of you.
It also doesn’t really make sense to do combat sites or mining in PVP fit ships. Ratting/mining is pretty boring, and I’d rather not doom myself to having to do 4x as much of it because I’m doing it in a sub-optimal fit, on the frankly non-existent chance I’ll get a decent fight out of it.
You also have to take into account the investments null groups have to put into reaching the levels of farming their members have access to. Lets take Delve as an example. (Since that seems to be what all of null is judged by.)
Goons have put crap knows how many trillions of ISK into the infrastructure that allows them their relative levels of safety and high income. Everything from sov structures to high-level citadels and manufacturing networks, to the injected combat toons and ships they use to enforce their safety umbrella. It took a serous amount of time and investment for them to set that up. Even smaller and less secure areas of null take a great deal of time and effort to set up and maintain.
In low-sec you basically just have to show up. Base out of an NPC station and you don’t even have to drop a citadel. No investment, no risk to anything beyond your own ship. It would make little sense for that to yield better rewards.