You're not just sending the raw data. You're also sending the address of the receiver, sender, a checksum, and often a few other bits of information.

If you send one megabyte of data, it will be cut up into a few hundred or thousand packets, and each packet will have extra information added to it (the stuff mentioned above). Then when you send it out over ethernet it'll have another layer of information wrapped around that. So you end up with:

|----------------------------------------------------------|
|           |-------------------------------------------|  |
| Ethernet  |         |------------------------------|  |  |
|   data    | IP data | raw data                     |  |  |
|           |         |------------------------------|  |  |
|           |-------------------------------------------|  |
|----------------------------------------------------------|

(not drawn to scale)

And that's ignoring what happens when there's a collision, or the IP layer gets congested. IP in particular was never meant for massive speeds.

One complicating factor, at least for some some types of data, is that what you're sending is often compressed on the fly. So it's possible to receive more data that 10Mbit/s, although the data rate on the wire will be slower.