I get your point, and was pretty shocked to find acme.sh, but after the certbot PPA made a giant mess of my system I gave it a try. On the other hand, why should I balk at running thousands of lines of bash, but be fine with thousands (or many more) lines of C, Python, PERL...? You can write crappy or beautiful code in any language...
Perhaps you wish to have "real" certs on appliances like F5s and Isilons (FreeBSD-based) where you can't install extra stuff, but where curl and openssl (and bash/zsh) are present.
Or perhaps you want to run simple software that you can actually audit. While "over a thousand lines of bash" may take a little while to examine, good luck auditing Zope, which is what certbot pulls in as a dependency:
Similarly with F5: there is (was?) no native ACME client (at least a few years ago when I first looked at it). So I download dehydrated and used various CLI interfaces to schedule automated runs and importation of the certificates.
There was no pre-compiled binary, and no compilers, on either system, and so talking about a "lightweight C executable" is non-sensical. Further, even if we (managed to) compiled things off-host, when we did an OS upgrade on either system, a whole bunch of libraries would change and we'd have to (remember to) re-compile. There is no such worry with a shell script.
If you want to have ACME-fetched certs on a general computer system, then compiling a C program (large or small) is an option. But there are scenarios where compiled/compiling C programs is not an option, and you telling me otherwise when I have personal experience of these situations takes some chutzpah.
Again, I have a better chance at reading all the code of dehydrated (which I have, in fact, done), then reading all of the Python code that certbot pulls in via dependencies on Ubuntu/Debian.
Sometimes you write in shell because it's the lowest common denominator.