Is it possible that a reduction in vegetation is a cause of global warming?
The short answer is that people do talk about it.  It is commonly referred to as "land use change".  In general, the carbon dioxide equivalent of the effects of land-use change is on the order of 10% of the total anthropogenic contribution to CO2 emissions.  This may seem small, but it is important to realize that land-based plants are only a portion of the carbon dioxide sinks. You can see below an image from globalcarbonproject.org that shows the relationship.  See here for full presentation.