1500 bytes is also the maximum size that can be used and have errors reliably detected by the particular CRC32 in Ethernet. And that's a hard requirement as sometimes the only way you notice a collision at the receiver is when you notice the packet failed the CRC check. A more complicated CRC was not an option because of the limits of hardware complexity (and cost) at the time.
That’s what I was told when I worked at PARC in the 1980s. Folks these days forget (or never learn) how Ethernet worked in those days.
(There’s no reason people should know how it worked — the modern work of IEEE 802 has as much relation to that as we do with lemurs).