Well, what else can it do, really? It has to boot with pretty much zero knowledge about the external world (maybe except asking the user for the current date and time). Sure, you can hardcode an outdated list of CAs (it's always outdated because the system can be booted 10/20/100 years after it was made) in but that just opens you to unexpected obsolescence, and you usually can't put too much stuff in the bootloader anyway.

Not really dissimilar from the human upbringing: leave a baby with "bad" guardians, and it will grow up corrupted. That's a feature, not a bug: if you knew what behaviour exactly you wanted (other than "whatever Simon says"), you would just bake it in in the first place, right?

If we pretend we're revising TFTP boot in 1995, let's have it get up to 20 boot options from the server and their md5 hashes, and if it's not set to auto it waits for the user to pick one. It then verifies the hash as it downloads. Also it uses TCP for the download.