since的使用方法

老師你好，在經濟學人看到一篇題目為：‘Time to end the academic arms race’的文章，其中有一點中有如此一句話：“As more young people seek degrees, the returns both to them and to governments are lower. Employers demand degrees for jobs that never required them in the past andhave not become more demanding since.”請問后半句have not become more demanding since這句話如何理解？其中since的使用方法是什么呢？謝謝你！

Employers demand degrees for jobs that never required them in the past and have not become more demanding since.
【翻譯】雇主們目前需要擁有學位的工作在過去從來沒需要過，此后也沒變得更苛刻。

since 可以用作介詞、連詞或副詞。單獨用就是副詞，后面不接賓語時，看作省略了已知的賓語（ then），即：since then。又如：

He left home two weeks ago and we havent heard from him since.
他兩星期以前離開了家，到目前大家一直沒他的消息。