Because the Senate gets to set its own rules for how it operates.
It's not like the effect of TPA would be a surprise to the Senate. Senators understand very well what it would mean for their ability to amend trade agreements. If they want the chance to review and amend the TPP, they won't pass TPA.
This is in itself a huge problem: Public debate in the legislature is not just for the legislature, but for the public. A fast-track enables back-room dealing between the executive and legislature that effectively cuts off the opportunity of the public to review and lobby against passage of an agreement, and substantially insulates them against public criticism of parts of agreements by giving them an excuse ("I could only vote yes or no") and keeping it out of the news cycle as much as possible.
In other words: Everyone, not just Americans, should worry even more about the contents of such an agreement if the US Senate chooses to allow such an agreement to be fast-tracked, because you can be virtually certain the reason will be because the Senate power brokers are worried the content would not withstand a longer public debate.
It's not like the effect of TPA would be a surprise to the Senate. Senators understand very well what it would mean for their ability to amend trade agreements. If they want the chance to review and amend the TPP, they won't pass TPA.