Why Speed and Duplex setting are not set to the maximum by default ?

I just installed ethernet instead of using wifi on my main pc. I was wondering why I was getting only 80-100 Mbps instead of the 250/250 my older pc get. I tough my cable was faulty

I found in speed and duplex it was set to 100Mbps and not 1gigabit. Luckily it fixed the issue now I'm getting 250/250.

But why it is at a lower speed by default ? Considering my older pc was an easy plug and play and with full speed.