I noticed your two links mention different unions striking; some further research might be important here. One of your links is not graduate students striking, but the union for skilled workers at UCSC. The other is a union-sanctioned strike during (I think) a contract negotiation year. Neither is representative of the assertion that graduate students strike every year.
Sure, not all strikes are officially in the name of the grad students, but (a) the unions act together, and (b) the grad students specifically have gone on strike 3 times since 2013.
Here's news on the strike from 2018: https://news.ucsc.edu/2018/05/strike-updates.html
Here's news on the strike from 2017: https://news.ucsc.edu/2017/01/strike-message.html
There's reason to believe that no matter what deal was reached, they would just strike again next year, to try to get more money. How much is enough?