It's the same reason why someuniversity.edu has a big Admissions link and a small login button for current students who are actually consuming the service. The website serves as a billboard more than as a production floor.
The website is telling you they don't value returning customers, all they are about is new customers.
When your UI team is testing user flows, if they're not starting from a fresh/clean session, they can fall into that pattern and forget to walk the "existing user, no auth" path.
At a past employer we ran into that with regards to an Apple/Safari issue. It's been a while, but iirc it was something to do with scroll heights, modals, and a render engine update in Safari maybe 3-4 years ago. Nobody caught it during testing due to us using manual testing in a variety of browsers, and it was live for almost 3 weeks. It was annoying at normal use, but if you had fonts increased (i.e. eldery/vision impaired user) it meant the user would be unable to login.
After that incident we switched to a hosted browser testing solution (I think it was BrowserStack, but might have been something else) and we made sure to flush things and test on multiple browsers per platform. I don't remember the full list anymore but it was something like 20 different browser/platform/version tests each.
I used to google "github", hit the first link, only to find out there was no way I can login, google again with "github login".
This was one of the worst UX I have ever had on the web.
Some replies mentioned that this was to maintain access to the pricing page for users, but if that was the case, having pricing accessible while logged on seems like the real solution.
From official replies, it appears that the real motivation behind this is marketing upsells for existing users/customers. Likely the reason they think this works is AB testing or some other conversion metrics, but I don't need stats to tell me that it's quite shady and annoying.
New customer = Very Important Person (TM).
Existing customer = Who cares, as long as they can't easily figure out how to cancel the subscription (NYTimes, looking at you)
I believe it is the reason, and have read so (somewhere).
Sometimes, it is worse on mobile where the login button disappears and you have to click on a "sandwich/menu" icon to display it; to this I say: guilty as charged. In this case, there are hundreds of other more pressing issues and we'll get to it. Maybe I'll change it later.
Man it annoys me.
But the reason "signup" is more important to a business/service than "login" is that it's WAY more expensive to GET a customer than to KEEP a customer
It takes more time to get a customer
It takes more pitch to get a customer
It takes more incentives to get a customer
To keep one, all you have to do is not piss them off so much they decide to leave
It's why magazines are filled with promotional 1-, 2-, and 3-year subscription cards at 60%, 75%, or even 95% off the newsstand price
Once you're in, you're in - and getting un-in is either difficult, or not worth the trouble
Those that were already sold to and bought in to the product and value it provides, shouldn’t use new visitor brochures to use the product.
Some products would have different funnel for the lifecycle of their product. Sometimes it might make sense to do one thing for a specific funnel and might not on a different context.
However, that assumes we are making decisions based on data.
Sometimes a team / company can will make decisions even if it is not optimal in terms of conversations because that's what they feel like doing.