Why is it necessary to measure the NMR frequency relative to a standard such as TMS? The horizontal axis of an NMR spectrum is "chemical shift". A standard environment such as tetramethylsilane is chosen as the zero of chemical shift. The frequency at which resonance occurs for other environments is then measured relative to the frequency at which resonance of the standard environment occurs e.g. a particular environment might resonate at a frequency 1ppm lower than standard environment.

Why can't we measure the frequencies at which resonance occurs absolutely, not relative to a standard environmnent? Then we would have a horizontal axis of frequency instead of chemical shift.
The use of an standard makes it possible to compare spectra. The frequency itself is dependent on the used magnetic field, while the the chemical shift is independent of the magnetic field and thus comparable using different spectrometers.