Sorry, but the long answer is really long. This isn't a plot by the cell phone manufacturers to force you to discard your perfectly good modem. To understand why, you have to understand how normal modems work and how they relate to the regular land-line phone system. Originally the phone system was a circuit switch, and modems were analog.
Time marches on, and the modems got faster.
There things stalled. Meanwhile, the phone companies had been switching to digital systems which were not cross-bars. Instead, they worked by digitizing the analog waveform being fed to them, packetizing the results, and interleaving it at very high speed into a TDMA bit stream inside the switch. So to connect two lines to each other, instead of a physical switch making an electrical connection between them, the switch tells each which TDMA time slot to use. This massively simplifies the switch and improves reliability by removing all the mechanical moving parts which were associated with the older cross-bar approach.