Some do. Girl Genius and Gunnerkrigg Court both update regularly no matter what the artist(s) are up to. I know Tom Siddell (Gunnerkrigg) is about a full chapter (~30 pages?) ahead of the current page and he does everything on the weekend since he also works full-time. I know less about the Foglios schedule (Girl Genius), but they’ve been in the business a very long time and offhand, I can’t remember them missing an update…maybe once? They did update for awhile only in B&W because their colorist was in hospital, but they still updated.
I also read Questionable Content, which runs like the webcomics you describe. I’d don’t mind, I find the Yelling Bird filler strips amusing. (Sometimes Jeph will do a few comics in advance of a trip, or at least he seems to do that more often now).
I think some webcomic artists stumble into full-time comic writing and if they didn’t start out with a buffer, it’s unlikely they’ll have one once their comic turns into a business. As long as their numbers stay up and fans buy stuff, there’s no reason for them to change.
Also the revenue is from different sources, for different things. Newspapers contracting artists for a daily/weekly strip vs fans buying shirts/prints/books. Fans (generally) don’t pay for to see the webcomic, so maybe there’s less incentive to make sure there’s a strip for each update day? And before shops like TopatoCo came about, most webcomic artists handled the merchandise themselves, so part of the day was spend boxing and shipping while the rest was for the comic.
I keep thinking of things! Arg. I wonder if conventions would make a difference. Most webcomic artists do several conventions a year - do newspaper artists do the same?