Why does anime always have to have 26 episodes?

12-13 episodes:
One season (example: Spring)
(One season duration: 3 months, 1 episode per week = 13 episodes)

24-26 episodes:
Two seasons (example: Spring and Summer)
(2 Season duration: 6 months, 1 episode per week = 26 episodes)

How much is epsodes anime will have depends on creators.
If they decide to make the anime with 25 episodes, this anime continue through two seasons... (example: Spring and Summer)
It's rare that anime ended up in the middle of season (example: Summer)

Ps. Sorry for my english, but I hope this will help you :)

Good question,I have no idea,maybe to see if poeple see it,if they do they make more,that's what I think

IDK. It's usually the older animes like Trigun (I think it's a 2003 anime) that have 26 episodes. Actually, the older animes usually had 25 episodes. Trigun had 26. The newer animes are much longer, well, except for Kuroshitsuji. Altogether, it has 36 episodes. From what I've heard, One Piece has over 400 episodes. Bottom line: I have no idea why the episode limit. I've heard of short and sweet, but IDK if those same principles apply with anime.

because when famous manga turns into anime it is only until half the story line it is the wraped up for 1 or 2 seasons for 24 or 26 episodes because manga is not as popular as the anime however if the anime gets extremely popular it is then stretched like a rubber band to make money though that is my opinion.

Who said anime with 12-13 episodes only had 1 season? Koihime Musou has 13 episodes for each 3 seasons....
On Topic: I don't know the answer to that totaly wrong question...