Well I chose not to be a series regular, so I don't get the gifts or the big salary. But I also have the ability to go out and do movies in the middle of the season, which is really nice. If you give your heart and soul to Buffy, then you give your heart and soul to Buffy. But I didn't want to be in that place because my real love is film.
Amber Benson didn't want them to.
SOURCE
Once you are part of the main cast of a show, you are contracted to that show and that means that you have to be available at any time during the filming period. (Usually July through March) Even if you're not appearing in all the episodes of that season, you need to be available in case of last minute script changes.
This makes it VERY difficult - if not impossible - to take other jobs during that time. Amber probably felt like she wanted the space and freedom to take other acting jobs as and when she wanted to. She was writing and working on Chance during S5, so she probably felt like she wanted the time to really give it the attention it deserved, so that she would be able to start filming it as soon as BtVS finished for the year.
By the time S6 rolled around, Joss probably thought that it would have a more emotional impact if he added her to the credits of the ep she dies in, and Amber said it was so that she could say a proper 'thank you' to the fans.
How was Sarah able to film movies and Buffy? She was filming Cruel Intentions during the 1999 Season of Buffy
Because she did it during hiatus - usually mid-March until the end of June. She colored her hair dark for that movie and it wasn't dark on the show, ergo, it wasn't filmed when she was filming BtVS.
There is NO WAY anyone who was a main character in a TV series would be able to film a movie during the months that the show shoots. They're all filmed during the breaks. A TV show - especially something like BtVS, AtS, Supernatural, Arrow - typically shoots 14 plus hour days, five/six days a week, July until March.
Not only would an main actor not have the time to go and film something else, but there's clearly - see Amber's interview - a clause in their contract that means they are contracted to that show and that show alone during the months of filming.
Interviews and appearances are a different matter - that's promotion for the show, and they probably have something in their contract that means they have to do a certain amount of that per season.
Was this the case with season 5? She was literally in the entire fifth season. She was only absent in some episodes in Season 4 and 6.
ffs she was a regular practically in season 5 from Buffy vs. Dracula to The Gift
they even did a Tara episode. Shouldnt she had been a regular by s5?
Well I chose not to be a series regular, so I don't get the gifts or the big salary. But I also have the ability to go out and do movies in the middle of the season, which is really nice. If you give your heart and soul to Buffy, then you give your heart and soul to Buffy. But I didn't want to be in that place because my real love is film.
I'll repeat Amber's words:
She specifically chose not to be a series regular. It was her choice. Whether that applied to her entire stint on the show, I don't know, but it was a choice that she made herself.
Perhaps Joss and the rest of the team wanted her to be a series regular, no one can really say. But the fact that she wasn't has nothing to do with Joss or anyone else. It was Amber's decision.