Bear in mind that Kontakt Player is the demo/trial version of full Kontakt, giving you the opportunity to see if you want to invest in the full version.
For the 15 minute time-out to be lifted, the author/publisher of the library needs to purchase a license from NI. There have been varying reports of the cost of this license, but I would imagine most people who give their time to making a free library don’t want to then pay hundreds of dollars so that people using a demo version of Kontakt can use it.
NI give you 50 instruments to start with to check Kontakt out.
Here’s the next best source for the handful of 3rd-party free player libraries (it is worth reading about the crossgrade);