Hi,

This reasoning behind this is far beyond MCAT level understanding of NMR, and is usually covered in a graduate NMR class.

You can have splitting in carbon spectra, but because carbon13 (the nucleus used for carbon NMR) is much less naturally abundant than 1H (the isotope used for proton NMR), we experimentally run proton decoupled carbon NMR spectra. This boosts your signal a little bit and makes the carbon signals more intense, resulting in a better signal to noise ratio and shorter data collection times. It also declutters your spectra because there can be carbon signals very close to eachother - making it confusing as to what is splitting and what is a carbon signal.

For the MCAT, just know that 13C spectra don't have splitting because we do something during experimental data collection to prevent this.