Because: "quality and engaging gameplay" reasons. Frontier want you to travel on your own, explore and discover. So any means that makes it more convenient is taken away. You want a new ship? Roam like a dumb in search for a station that sells it. Luckily we have things like EDDB that tells us exactly where this ship is and the nearest location it's available. You want it for a 15% off? Unofficial Wiki has it, all you have to do is to plot a route there.
The problem starts with you having now 2 ships. Example, I bought new Krait over 300 LYs from my home base. Flew there with Krait, at dealer I left my FDL. Now I have two options:
- pay 670k for FDL door-to-door delivery
- spend at leas an hour flying in some small disposable taxi ship to get FDL back and bring it back home.
And this is exactly how Frontier wants it. "Convince" you to actively fly between locations or face the high transfer fees. Which at one point you would want to cover with some missions that take time. In the end you spend time and this is how this game was designed.
As for 2: Bl1p pretty much covered the topic. NPCs are created for you and deleted when not used. This can lead to strange things like "instant healing" after combat. You have an assassination mission. You spot the target, engage it but he escapes into another system. You track him down and engage again to see he's at hull shield and health, despite you managed to strip his shield and reduce his hull to 15% 10 seconds ago.
This is because game by "escape" mean "delete NPC from instance" and when you spot him again it's brand new spawn. Travelling with NPC could be done but they don't have fixed routes, they are just background noise.