What reason is there to believe that it was always the same?
Were you around to observe it?
The fact that I was not around to observe it is not evidence that it was different at one time. Your question amounts to the demand that I prove it didn’t happen. I can not and will not attempt to prove a negative. Again what evidence is there that the speed of light is variable? I would like a direct answer please.
General Relativity.
What have you got?
There are a lot of things that depend on the speed of light. Such as what the spectra that different atoms would emit if they were heated. Yet the spectra for distant stars (that is, the light that was emitted long, long ago) has recognizably the same patterns as what we see today, except for a Doppler shift that we understand as having to do with the expansion of the universe. If the speed of light changed during that time, what spectra would look like back then wouldn’t match up so well with what we see today. Furthermore, if it once changed, why should it be so stable today? If can be measured so precisely the speed is now defined; that it doesn’t vary to 15 decimal places can be ascertained today. To within that tolerance, it does not.
The “operation” of matter and life, down to the atomic level, depends on certain constants.
Change those constants, and you change the way the Universe works. Including unimportant things like chemistry, and electrical conductivity. And outside of a surprisingly narrow range. . . life is impossible. . .