Why don't airlines use n numbers as callsigns?

To add to the answers, when you call in with a N you usually say, Cessna 233MA is ....... or twin cessna 665YY is...... but with a Boeing you called in Boeing 787BA is .... no one would know what it is, a 777, 737, 717, or a 787. Call signs also help minimize confusion. If you had two airbuses with two different airlines, it's easier for the pilots to know who ATC is calling if Frontier 182 is called versus airbus 7FR. Also, a lot of airlines us the same last two letters, if you have two boeings with N769UA and N889UA you have to "Boeing 9UA" which is confusing. It just eliminates confusion among pilots and controllers. And it's cool, like Redwood and Waterski, or Brickyard. They are cool callsigns!

It helps to distinguish a Boing 747 from a Piper Cub.

It is easier all round, a pilot knows he is flying a service which is Speedbird 38, he may not necessarily be aware that this service is to be flown by any particular aircraft of the 40 or so B777s in the BA Fleet until he boards the aircraft.

Controllers can also have the same issue, they know that the flight they are controlling is BA 38 and they know it is an airline service, they don't necessarily recognise N12345W as being that flight, but BA 38 can't be anything else but that specific flight, it avoids confusion and makes it certain which aircraft is being controlled

because repetitive /recurrent callsigns help you with quick judgement about that said flight's mission and destination.

the ATC does not care which of g-abcd or g-fuzz operator sent for the route.. the ATC needs to know that that's the Speedbird (ergo, a large jet with lots of people flying)

when i "introduce" to an ATC as Christopher, ANYONE on radio knows the station calling [me] is the medical flight, much like i can visualise the airspace based on airbase- typical callsigns in the vicinity.

Because on occasion airplanes are changed on short notice. If the tail number was used flight plan information would need to be changed increasing the possibility of an error. Using flight number nothing changes as far as ATC is concerned.

Too complicated, the callsign conveys more information (company, routes are standard, etc.)

Because ATC wants to know about the flight route, destination, etc. they usually dont care about the airplane itself

because one plane can fly many different routes/places in a day and it's easier to track them w/ a specific flight number for that destination

To make everything confusing in Hatsfield in Atlanta where you have to say 'David' instead of 'Delta' because of all of the Delta flights.

because it tells everybody - other pilots, atc - useful information.

exactly which plane it is is irrelevant. the route it's flying is very relevant.