This whole move is about corporate governance. The US makes it really easy to start or manage corporations and the courts are (mostly) streamlined and predictable, especially the chancery courts in Delaware. Cayman Islands adopted much of Delaware's legal approach to corporations in 2016 to make the island more business friendly rather than just a tax haven, and they've got a foot in the Latin American market. Singapore is the SEA equivalent of Delaware.

Nothing else much to it. In reality they're all going to have to register to do business in Canada/California/whatever and pay their taxes anyway. Structuring the parent in one of those jurisdictions just makes the legal wrangling about ownership and stock classes safer and more predictable to both investor and founder.

This exactly. Canadian common law has some very odd implications for corporate governance. Much higher risk of governance deadlock due to recent rulings.

VCs are not going to know that when evaluating a company. YC as the incubator and the first check in has an obligation to vet the situation for future investors. The easiest way for them to do that at scale is to ensure they are experts in a very small number of jurisdictions that are predictable.

Honestly, it makes sense.