Question

If a wireless card can send packets, why can't it inject packets?

All have to do to inject a packet is create a raw socket, construct a packet with the spoofed headers and then send the packet through the raw socket. This would require root/admin on most operating systems, but it would still be possible.

Some people give the argument that both reading/injection have to be happening simultaneously but you could just receive packets, parse data, stop reading and inject packets when necessary.

So why do some wireless cards not support packet injection, is there another limitation?

Answer 1

The term "packet injection" usually means interfering with a established session between two other devices, for which you need a driver that supports monitor mode.