A cellular modem doesn’t take up much space and doesn’t require any special engineering. It’s no different than adding a WiFi card or supporting external cellular USB adapter.
Apple is still a publicly traded company, and they make things that they think are great AND will sell.
Case in point, iPods, mac mini, iPad mini, etc. Apple is willing to focus and prioritize certain things over others, and obviously they don't think LTE on Macbooks is in the upmost importance.
But then again, once they are making an Ax powered Macbook, they might just make LTE models ala iPads as the SoC and board are already designed to work with cellular.