Because, for the 1384th time, they're pretending they can ignore what the programmer explicitly told them to do
Creating memset_explicit won't fix existing code. "Oh but what if maybe" is just cope.
If I do memset then free then that's what I want to do
And the way things go I won't be surprised if they break memset_explicit for some other BS reason and then make you use memset_explicit_you_really_mean_it_this_time
Your problem is not the C committee but your lack of understanding how optimizing compilers work. WG14 could, of course, specify that a compiler has do exactly what you tell it to do. And in fact, every compiler supports this already: Im most cases even by default! Just do not turn on optimization. But this is not what most people want.
Once you accept that optimizing compilers do, well, optimizations, the question is what should be allowed and what not. Both inlining "memset" and eliminating dead stores are both simply optimizations which people generally want.
If you want a store not to be eliminated by a compiler, you can make it volatile. The C standard says this can not be deleted by optimizations. The criticism with this was that later undefined behavior could "undo" this by "travelling in time". We made it clear in ISO C23 that this not allowed (and I believe it never was) - against protests from some compiler folks. Compilers still do not fully conform to this, which shows the limited power WG14 has to change reality.