The reason for that - in fact, the reason 2.5G/5G exist at all - is to get a full 100 meter run out of Cat5E or Cat6 for Wave 2 AC access points at higher than gigabit speeds. Replacing APs is pretty simple, replacing the inside cable plant for an office environment is not. Since 10GBaseT requires Cat6A or better for runs to max length, that was the only option to go above gigabit.Lastly, some mainstream routers with WiFi6 now come with 2.5 or 5 Gbps RJ45s to support that new standard and bandwidth (why they don't go straight to 10Gbps, I don't know...).
For more historical information on it, google NBaseT.