Firstly, filtering is an overtaking-style manoeuvre. We overtake on the right here.
Secondly, many roads have a solid white line up the left side - Know what that means?
Further - Driver is on the right hand side of the car, travelling anything up to 60. The last thing they'll be looking for is a vehicle passing them on the left. Cyclists, perhaps, but that's it.
They'll be watching for bikes in their right mirror and may even hear your pipes, in which case they move left and... BANG.
Also, HGVs are fecking huge. If you can see safely past them to pass on the left, you have x-ray vision and belong in the X-Men, not here on the road. This is why cyclists die so often.
Lastly, the left side is where all the road crap, gravel, oil, dirt and shit gathers, as well as having anything from grass verges, to surface water drainage gullies and a higher percentage of slippery metal manhole covers.
You still wanna ride up the left?