I'd arrived at it by estimating that the average person will end up comparison-shopping for about 100 different products or services during the time that they hold a job, but will have only one job. If you assume 2 years at a job, that's about 1/week, which seems reasonable, counting all the gifts you buy, restaurants you visit, trips you plan, service providers you look up, products you evaluate for work, etc. Wouldn't be surprised if it's even more.

For any given business trying to optimize their website, it's a lot easier than that. Just look at the traffic flows in Google Analytics. If everybody hits the landing page and immediately clicks on "Careers", maybe the focus of your landing page should be on the job opportunities available. If they don't, you're probably right to focus on the product. If a significant number of people visit "Careers" but not enough to move it to the front page, you may want to exclude visitors to it from your remarketing campaign. (A sibling comment indicates that this is trivial for both Google and Facebook ads.)

That more sites don't do this - when it takes all of about 15 seconds to diagnose in Google Analytics and a couple minutes to fix - is probably a good indication that the economic losses from this situation aren't all that significant.