In these countries, people drive on the left side

Left-hand traffic is a historical standard

Even in ancient times, the left side of the road was the "right" one. One led their horse and thus also the drawn cart on the left side. There were practical reasons for this at that time: Most people are right-handed and thus one could hold a sword against an enemy with the free right hand. There was also a similar reason at sea: the rudder was operated with the stronger right hand and thus the helmsman sat with his back to the left side. Oncoming ships were therefore much more visible on the right.
The great shift to right-hand traffic started in France at the time of the French Revolution (end of the 18th century) and was driven forward by Napoleon Bonaparte in particular. On his campaigns, he introduced right-hand traffic in many conquered countries.
So it is not the British who want to impose left-hand traffic on the world, but the French who extended right-hand traffic over their numerous colonies.

Differences in the car

It's not just the steering wheel that changes sides when driving on the left. Problems arise with the gearshift, which is now on the left. Those who have operated the gear stick with their right hand for years now have to learn to engage the gears with their left hand and often get lost in the gearshift positions. The levers for the windshield wipers and turn signals are also swapped. The individual pedals, on the other hand, are not swapped. The gas pedal is always on the right, and the brake and clutch are always in the same position.
It gets confusing again with the "right before left" road rule: As expected, this rule is often reversed in countries with left-hand traffic. So, it's "left before right" - but that doesn't apply in all countries.

Left-hand traffic as a safety aspect

Safety researchers often advocate left-hand driving precisely because of the steering hand. If you shift gears with your right hand, your left hand, which is usually weaker, is alone at the wheel at that moment. It follows from this that right-hand traffic is a little less safe. If you take a closer look at the statistics of the last 10 years on fatalities in road traffic, this view is confirmed — albeit with only slight differences. Across all countries with left-hand traffic, the annual fatality rate was 16.33 deaths per 100,000 inhabitants. In the countries with right-hand traffic, it was 17.73 deaths.

A changeover is expensive

When a country switches from one direction of travel to the other, this is accompanied not only by changes in mindset, but there are also enormous costs involved. Traffic lights are not simply reversed. They have to be switched differently. At major intersections, all lanes have to be redesigned and re-marked. Even parking spaces that run diagonally to the direction of travel must now run in the other direction. Streetcars cannot simply be run in the opposite direction, but must be given a new network in wide areas. In Sweden, this led to the closure of numerous lines when the country switched to right-hand traffic in 1967.
Last, but not least, car manufacturers must also adapt to the country's new conditions. Bus companies that used to let their passengers get on and off on one side now have to allow them to do so on the other — but there are no doors there. In addition, import restrictions are often imposed to gradually adapt the vehicles to the future direction of travel.
Samoa introduced left-hand traffic "for economic reasons" and despite massive protests in 2009. The small island nation in the South Pacific had a population of just under 180,000, and there weren't even any multi-lane highways for the 18,000 cars on the road at the time. The changeover cost more than 250 million dollars — more than two thirds of the gross domestic product.