Question

it's high time that we did sth.这里为什么要用虚拟语气？

it's time to do sth,这里为什么不用虚拟？

Answer 1

这里没有用虚拟啊。虚拟一般是“would+动原" ， 这里只能说从句中用（一般）过去时，这是和文章的时态保持一致的。
这里是一个句型：it's high time that sb do sth 是某人做某事的关键时刻了。
相似句型还有：it's high time for sb doing sth 是某人做某事的关键时刻了。

Answer 2

①It is high time to do sth.=It is time to do sth.
②It is high time (that) sb. did sth.
③It is high time (that) sb. should do sth.
④It is high time for doing sth.

有这几种用法。2最常见