Lasers are easier to setup (you just need two end stations) and don't need dedicated lines (you don't have to lay any fiber). If you have an existing connection between two points, then fiber might be better / same, but if you had to setup something for cost / speed, then laser would win.

The higher the index of refraction of a medium, the slower light travels in it.

Air barely refracts light, and glass refracts it heavily.

BTW laser links are prone to work worse in bad weather. Microwave links are often used instead.

Yes this is why many high frequency traders have installed laser or microwave links to their nearest stock exchanges, simply to gain a few microseconds advantage in their trades. Fibre isn't as fast as laser going through the air. It's 50% slower (as the parent comment states) (unless you are using special hollow core fibres which are uncommon for now). More importantly, fibres rarely go in a straight line between 2 points, they wind their way through buildings, down into basements, through buried pipes etc and this all adds extra distance to the route and hence more delay to the signals. A line of sight link is the shortest route between 2 points.

That question does not really make sense. Fibre links use lasers. Maybe what you are asking about is free-space vs fibre links. Yes some HFT players use free-space optical or microwave links, because of the improvement in latency, they typically also eliminate all (or most) FEC which introduces significant latency as well. However, they don't really care about data rate so much.

For almost all other applications the factor 1.5 you gain from free-space vs fibre is not worth it. A trip around the equator in an optical fibre takes 200ms. That is acceptable for almost everything. Moreover, the latency of using any something free-space would likely not be much better, because one would need to regenerate at least 3 times (which might add ~10ms or so for each regeneration)