Why do mosquitoes hover over our heads?
In many mosquito species (and other diptera) males make swarm to attract the females. The vast majority of individuals in swarms are therefore males (but there are always a number of females in them as well).

These swarms are a type of lek mating which is also very common in midges (thanks to @arboviral for this info). Are you sure those you see are mosquitoes and not midges?

In order to produce these swarms, they typically hover a recognizable element in nature to swarm around, such as a small tree, a bush, a big stone, etc... On a soccer field, players are a distinct element to swarm over!

Of course, females are attracted to $CO_2$, warm bodies and odour of sweat but this is IMHO likely not an important factor causing the swarms you see. See @Kiny's answer on the subject.

References:


Takken et al. 2006
Yuval et al. 1994



  Also, why is this in the evening?


Mosquitoes typically start mating at the beginning of the evening and then forage during the night. However, this varies quite a bit from species to species. Aedes egypti for example, forage day and night while species of the genus Culex forage only for a few hours after the sun has set.

Such variation in the timing of mating behaviour can lead to (pre-zygotic) reproductive isolation between lineages. Potential reasons for being active at specific times and not others include temperature management, easier to parasite for females during the night (only females bite), timing with flowers (as both males and females feed on nectar). I would not be able to argue for why a specific species shifted its circadian rhythm in a specific way though.