I don't think anyone is suggesting never to outsource, or even that the answer to "build vs. buy" is always "build".
The point is not having a critical dependency on a single vendor.
I don't think anyone is even saying "never", either.
The question is one of risk.
The risk of your tax accountant "going down" (or being crooked or something) is sufficiently low, and the cost of making that redundant is sufficiently high, that I don't think anybody does that. This could even apply to general accounting software, though one might argue that's not critical in the same way and/or more easily replaceable.
There's a sibling sub-thread that essentially implies, if not outright states that there's no choice but to "buy" in "build vs. buy". Although it may be true that building is no longer a practical choice due to time/cost, under certain circustances, it by no means eliminates the risk if there's only one vendor.
It's also a decision that can end up having tremendous costs at scale, if not re-evaluated, especially with even a soft version of vendor lock-in, like AWS.