Function calls have an overhead, and if the function is only used once it makes no sense to break it out.

It also makes no sense to have a rule on the maximum number of lines for a function. These rules are often created by people who don't write software that needs to have high performance.

One of my professors used to encourage the heavy use of helper functions that just... well, like in A or B in carmacks article, break your code into chunks with clear names that can be unit tested.

How common is automated testing in AAA games?

function calls push registers, allocate a stack frame, and push a return address. By breaking your inner loops into more functions you've now generated megabytes of pointless memory thrash... on a console. Additionally, at the time this stuff was written, compilers and toolchain weren't neccessarily standardized, or were customized to accomodate certain use cases, so they needed more hand-holding to generate performant code. This is just a fact.. I'm not saying you're wrong, but you're definitely viewing this with hindsight.

With modern C++ you could do this with a lambda

...which is NOT high on the list of concerns for a game title, what you want instead is massive performance gains, so you can do more with less, and in the end produce a better experience than your competitor. -- function calls/indirection/making your code 'easy' to understand, all have a RUNTIME cost, and many small costs add up to a large cost, the reasoning is really that simple.