dd command... we've all used it, but why a limitation?

This is not a support request.. just a curious question. I swear.

I've used the dd command numerous times. You wget an ISO file, then use the dd command to write the image to a usb disk. I attempted to skip the download step and try to write directly from the source URL to my usb drive and it doesn't like it. Anyone know why this doesn't work? I'm supplying the source and destination. Simple enough.

I get that download speeds may may slower than the dd write speed, but still, why is this not possible?