I'd be very interested in the specifics of how and why late stage investing is so different that the success of YC in early stages doesn't translate.
1. Breaking focus / competing on multiple fronts. Lots of firms specialize in A-stage or later. By investing in seed and later rather than just seed, the later stage firms see you as a "competitor" for, rather than a "supplier" of, early stage startups. You have that many more competitive relationships rather than cooperative ones.
2. LP fundraising. LPs have to make choices as to who to fund, especially in this economy. Later stage vetting, returns, etc are different than early stage. May not be worth the heavy lift of competing for LPs.
3. Specialization. Once you get into later stages, you get firms that specialize by industry vertical. Not just business (marketplace, fintech, hardware) but even within software (SaaS, dev tools, consumer, enterprise, etc). Might make it harder to make deals. You now have a multi-front problem where each potential counter-bidder for the deal lead has hyper specialization to the startup, whereas YC is a generalist by nature.
4. Competition for deal terms. Most of the time, the deal lead sets the terms. If you can't aggressively bid to lead deals, they may not get the best economy for each of the deals since the lead may have other priorities. This may produce less optimal returns vs just putting more money into seed.
5. Partner / investor preference. VCs compete for partners / investors. If partners in the late stage at YC are limited to only YC companies vs the whole late-stage market (or have other limitations), it may not work for them vs going to a firm with less terms.
Ultimately as a generalist investor, pre-seed/seed/A-and-later are very different markets. With interest rates this high and everyone being more picky, it becomes harder to outperform unless you can operate in that market independently. I suspect YC looked at a model for their returns and came to this conclusion.