LED filament lamps, are made from glass, as they are filled with a noble gas to cool the LEDs. So the gas probably reaches temperatures that plastic would melt. Also, these lamps intended to looks like the classic incandescent lamps, so the use of glass rather than plastic is also a reason for here.

Also retooling a factory is expensive, so if you only change a filament material you don't need to do much retooling..

I'm also pretty sure the plastic wouldn't fully block the UV-B from the mercury discharge, effectively resulting in a germicadal lamp, (those are made of I THINK quartz, someone please correct me if I'm wrong) which would be very unsuitable for general use for obvious reasons. 

Even if it did workout, and the plastic tube was successfully made, (not possible, at all) after long-term
usage of the plastic fluorescent tube, the tube could become yellow and melt over time due to heat.
I'm pretty sure at EOL the lamp would catch fire.

I'm also pretty sure the plastic wouldn't fully block the UV-B from the mercury discharge, effectively resulting in a germicadal lamp, (those are made of I THINK quartz, someone please correct me if I'm wrong) which would be very unsuitable for general use for obvious reasons. 

The plastic does block the UV even better than glass.
But the result of its absorption is its the well known corrosion (the browning,...).
And the additives, which are designed to make the plastic UV resistant do nothing more than block the UV directly on the surface - so it won't pass the wall either...