Why doesn't the boot messages persist in the terminal after booting, on modern Linux distributions?
Yes, I know I can make the boot messages persist with --noclear or TTYVTDisallocate=no.
Like described here: https://askubuntu.com/questions/58097/how-can-i-remove-the-clear-screen-before-login
However, why is this the default behavior? What is the rationale behind it?
It is also not true that you can just read the system logs if you want this information, or type dmesg. Some information is not always present in dmesg or the system logs, and require special debugging steps to obtain. Moreover, it is a mere inconvenience to have to go through these hoops just to read a simple boot message, as evident in this user's frustration (and mine): https://askubuntu.com/questions/58097/how-can-i-remove-the-clear-screen-before-login