Well you can tie it to TPM to store your encryption key which should only produce the key when the boot parameters match the key. This is what Windows already does but its not fully supported under Linux and somewhat insecure as you can't encrypt the initramfs (so someone can infect boot process there instead).
There are ways to solve that issue. But I think that you're correct, pinpointing the core issue with popular Linux distributions. It doesn't have to be this way, though.
1. You can sign and verify initramfs, it's supported by bootloaders.
2. You can merge kernel and initramfs into UKI and sign the whole image.
For UKI I imagine a big hurdle is the size of the images (given /boot/efi is usually only big enough for bootloaders, not kernels and initram) and custom kernel modules (e.g. Nvidia).
There was some systemd work on a spec for a boot partition to extend the efi partition for storing kernel images and initramfs, but I don't think any distro currently defaults to it on.
I think hibernation is currently also broken, since the hibernation file is stored unencrypted by default and thus can't be used with secure boot.
