RJ45 is/was the pinout/wiring of an 8P8C[1] (eight pin/eight contact) connector designed for the telephone system.[0] The wiring specification of Ethernet is ANSI/TIA-568[2].
The only thing RJ45 has in common with Ethernet is that it used the same 8P8C-sized connector. But even then, RJ45 had a key notch on the side and Ethernet does not.
IIRC, that was one of the reasons we had the center pair available with Ethernet for such a long time:
A given jack on a wall in a given office could be wired for a regular single-pair phone, or for Ethernet, or for both -- even at the same time (using the same cabling), as a design intent.
Of course this doesn't work with the gigabit spec we all wound up using, but it was never a popular option to begin with (and nowadays, we usually have VOIP if we even bother with desk phones at all).
The original idea was to have the first pair in the middle, the next pair outside of that, etc.. so that a smaller pair plug could fit in a larger sized jack.
However when it came time to specify the pinout when designing ethernet over utp they found the separation required to place the wires like this was too great and degraded the characteristics of the line so they placed a pair on each side keeping the center as is for some sort of compatibility. Like said you could have a phone line in the center of your ethernet. suck to be you it you did that then later wanted to upgrade to gigabit. It also suck if you had some idiot doing the wiring who had the bright idea "herp derp, two pairs of this are not used... I could run two ethernet lines over this cable"
Which is sort of my next complaint, tia-568, the intent was to have the A standard be used with the B standard for compatibility with existing phone lines . Every place I have ever been they just used the B standard. I mean it is not a huge deal, but it always bugged me.
The antiquated Registered Jack standard is RJ45S, which always gets butchered to RJ45 in common parlance [and this sans-S version has never actually existed].
For RJ standards, an S suffix signifies that it is a "single line" circuit.
The only thing RJ45 has in common with Ethernet is that it used the same 8P8C-sized connector. But even then, RJ45 had a key notch on the side and Ethernet does not.
[0]: https://en.wikipedia.org/wiki/Registered_jack#RJ45S
[1]: https://en.wikipedia.org/wiki/Modular_connector#8P8C
[2]: https://en.wikipedia.org/wiki/ANSI/TIA-568