Welcome to Android Central! Realistically, there are hardly any laptops of any kind that come with 3G or 4G capability. It might be that they're trying to avoid the stigma of those old terrible Windows netbooks. But more likely, the manufacturers probably figure that the majority of Chromebook users will be using them on a wi-fi network, and those who need to be mobile can just tether off their phone. Leaving a mobile radio out obviously cuts costs, which is important since one of the big selling points for Chromebooks is how inexpensive they are for what they provide.
I have an older HP Chromebook 14 that is 4G HSPA+ capable (and that also came with free 200 MB/mo of 4G data from T-Mobile for life)--you might be able to find refurbished or used units for sale. It's a decent Chromebook (with a Celeron 2955U processor, which is still pretty fast when compared to current Chromebooks), although it's only 720P.