Others have pretty much mentioned it. Too thin of conductors for the total length required which can overload the cable and heat it up. If you’re just charging your phone it’s unlikely to cause a problem but the more amps you pull the riskier it gets.
Here’s a helpful chart…
Edit: Even at harbor freight (cheap hardware store) a 50ft 12 gauge extension cord is about $40 and weighs 7 pounds.
I’m in the minority I guess cause I was happy about this. I hated having to install iTunes just to back up/restore phones. I wanted the full encrypted backup. iCloud doesn’t do that.