Electronic – In MOSFETs why is the gate-body inversion layer voltage able to create a conducting channel, yet the same source-drain voltage not

There are a couple of issues that you need to be aware of.

First of all, you need to make sure that they share the blocking voltage equally. Just like with diodes, you need to put high-value resistors in parallel with their drain and source terminals such that the current through the resistors is on the order of 10× the reverse leakage current of the transistors (or diodes). This will equalize the voltage drops to within 10% or so. Also, the speed of the individual devices might vary a bit, so it's also a good idea to put capacitors (typically 1 to 10 nF in line-frequency applications, less at higher frequencies) in parallel with them as well, to tame the transient behavior.

The second big issue is that the gate drive to each transistor needs to be relative to that transistor's source terminal; they can't share a common connection. Depending on the application, you might use transformers, optoisolators or specialized high-side driver ICs to allow each of the gate drive signals to float relative to all the others.

The electrons come from the source, which is an N-doped region. This source quotes Chenming Hu's book "Modern Semiconductor Devices for Integrated Circuits":

...there are few electrons in the P-type body, and it can take minutes for thermal generation to generate the necessary electrons to form the inversion layer... The inversion electrons are supplied by the N+ junctions...

I also ran across a (probably illegal) PDF copy of Ali Niknejad's book "Electromagnetics for High-Speed Analog and Digital Communication Circuits". I'm not going to link to it, but here's the relevant quotation from section 2.3 regarding MOS capacitors:

In the above discussion you may have wondered, “Where do the electrons come from to form the inversion layer?” In the body of the MOS-C structure, electrons are minority carriers and few and far between. So when inversion occurs, where do we find all the electrons necessary to invert the surface? Well, there was a subtle assumption that if we apply a change in gate voltage, we wait long enough for thermal generation to create a sufficient number of electrons to form the surface layer. We may have to wait a very long time! In other words, if we apply a fast enough signal to the gate, there isn’t enough time for the minority carriers to be generated and thus the capacitance remains at the low value given by depletion.

While the depletion region can respond very quickly to our gate voltage since it is formed by majority carriers, the minority carrier generation is slow. There is a simple way to solve this problem, as shown in Fig. 2.25, where a n+ grounded contact is placed adjacent to the gate. Normally electrons are prevented from entering the body, like any good pn-junction. But as we raise the surface potential, electrons can easily diffuse into the surface of the structure. Since the energy distribution of electrons in thermal equilibrium is exponential, changing the potential barrier linearly results in an exponential increase in the number of electrons that can cross the n+-surface junction and likewise an exponential increase in surface conductors.

I redrew Figure 2.25 to illustrate what he's talking about. You can see how similar this is to a MOSFET:

In an NMOSFET, when \$V_S > V_B\$, you get a secondary effect called the body effect or the substrate bias effect. This effect acts to increase the threshold voltage. According to Wikipedia, the body is acting more like a second gate.