Because the servers can only handle a certain amount of players at any one time. You are not the only player in the game, so it's just something that has to be dealt with (and speaking as a player on Balmung, traditionally the most congested server, my login queues are never really longer than... ten to twenty seconds usually.). If minutes are going by and it's not logging you in, then perhaps try playing at a non 'peak' time of day for your data centre area (aka: late afternoon, early to mid evening til about midnight in that data centre's region).

It could be so much worse, it could be how login was like at EW's launch in late 2021 with 5000+ players in the queue and waiting literally hours to advance for weeks.