Motorways are the safest because there is a barrier between the sides so effectively it is a road where everybody is going in the same direction. It's nothing to do with going the same speed - there are no roads where people are driving at vastly different speeds, anyway. Rural roads tend to be the most dangerous because of the combination of no barriers between the directions plus speed plus poor visibility in many cases. Overtaking is much safer on motorways as you aren't driving into oncoming traffic.
It doesn't encourage unsafe lane changing to use the right two lanes to overtake - if people only ever changed lanes in order to overtake, then it would actually be much safer. The problem with lane changes is when they are unpredictable, which tends to happen when people dodge between lanes based on where they think they can drive faster, this is a really dangerous practice. Accidents are less likely when you can guess what other drivers are likely to do before they do it, they tend to happen when people are taken by surprise. If proper lane discipline is adhered to then driving behaviour tends to be very predictable.
It would not be safer for all traffic to be evenly split across the lanes for this reason - it might be more efficient, arguably, but not safer. As other people have mentioned, some vehicles are speed limited, such as trucks, horseboxes, and some classic cars. These vehicles should stay in the left lane so that other drivers can overtake them safely.
Europe does have more driving deaths than the UK but most European countries clock up more kilometres on average than the UK too, we're a small country so don't drive as much. When this is accounted for the figures (especially in North/Western Europe) are not massively different - it is Eastern and Southern Europe where the driving deaths are significantly higher despite comparable distances being driven.
Personally I feel very safe on Dutch and German roads in particular. Dutch roads everybody just seems to be really nice and obeys the rules without really quibbling about it, which is very relaxing - possibly a bit too relaxing as Dutch roads are also very long, straight and flat. German drivers have a huge thing about lane discipline (you learn rather quickly never to sit in the outside lane there) but aren't so bothered by speed, of course supported by many motorways having no speed limit, however, this seems to work. (You can still be prosecuted for causing an accident while driving at higher than the recommended speed of ~75mph). France can be a bit hairy as drivers have the tendency to pull out suddenly with no notice, indication, or seemingly any logic. Belgium seems to have an unwritten rule of "if you want there to be a space/higher speed limit, just pretend there is one and act accordingly" which can be a bit alarming. The UK is not terrible but lots of people ignore both speed limits and lane discipline which means you end up with quite a jumble which is confusing and can be frustrating to navigate.
If you want to look you can see road casualties by kilometres driven here (the sorting doesn't seem to work properly) - the UK is in the lowest group but is also joined by the Nordic countries, then you have central Europe not far behind, with Eastern European countries having much higher rates.
en.wikipedia.org/wiki/Road_safety_in_Europe