Phone modems and digital subscriber lines (DSL) use the same kind of phone line to transfer data. Why is DSL so much faster than phone modems?

Because data must be converted to an analog audio signal. So that it can become transferable.

Explanation:

Digital Subscriber line (DSL) : - It is used for transferring data to small business and homes over the pre installed copper telephone lines.

Phone also uses phone lines but Phone modems are cheaper than DSL because the transfer rate is very slow because first the data must be converted to an analog audio signal. So that it can be transmitted.