Because the radio is not at absolute 0 and inside of a Faraday cage ;-)
Seriously, the S-meter accurately reflects the integrated (sumed) RF power of the individual FFT bins inside of the slice receive filter. There is always RF power (energy) being digitized by the SCU even with an antenna disconnected. There is an ambient level of environmental RF noise, cosmic RF noise and other RF signals. The only way to eliminate these noise sources is to cool the receiving device down to a temperature near absolute 0 and block outside RF energy using a Faraday cage (hence my joke to open this post).