This is why

剩女
In China. Older unmarried women over 25 years of age was called "剩女".
In society, a lot of pressure.

In China, [comma - not a period] older unmarried women, over 25 years of age, are called 剩女.

use "are" because the verb refers to the subject (noun) "women"

In Chinese society, there is a lot of pressure.

I suggest you add the word (adjective) "Chinese" because in many societies like Canada and the USA many women do not get married or have children until they are in their 30's.
Nowadays, in China, many young women need to use their time and energy to study, start their career, get promotions in their company, or start a business.  They do not have enough time to have children and take care of a husband. Am I right?

"there is a lot of pressure".  English uses "there is or there are" just to say that something, e g. "a lot of pressure", is exists and is true/ is a fact, e.g.
- there is only one city called Shanghai
- there is only one you
- there are many people in China
- there are many museums in Xi'an
- there are people from over 105 different nationalities who live in Canada.