The problem is really that the pit crew in LFS only work on one thing at once. Refueling takes rather longish too. Compared to RL LFS pit stops take very long. In RL they can change the tyres and refuel in around 5s, where as in LFS that would last over 20s. So it's more benefical to have a full tank and not waste time fueling than being a bit faster and then refueling. Same goes for tyres too, it's benefical to make tyres last for the whole race even if it means you will drive a bit slower overall. This really makes having a pit stop with actual work done something you want to avoid.