It is trivially easy to boot from a USB pendrive; most computers will do it by default if there's no preinstalled OS, many devices have a hotkey you can press/hold which redirects towards booting from USB, and even Windows itself will outright let you do it from the Settings app. You don't even have to interact with the system firmware setup program.
On the other hand, disabling secure boot (or taking ownership of it) is designed to be as difficult as possible, intentionally. Many vendors will actually have a Scary Boot Prompt or the like which makes you double-acknowledge when you change Secure Boot settings, and even reminds you (on every boot) that you have Secure Boot disabled.
It is also unfair by itself that MS OSes will boot without requiring the user to fiddle with the system firmware, but non-MS OSes won't. This puts a glass ceiling on non-MS operating systems regarding how easy their setup flow can be.