为什么英语中总是运用被动态？

你好！主动语态和被动语态视场合而定。
被动语态及用法
一 概念：当句子的主语是动作的执行时，谓语的形式叫主动语态；当句子的主语是动作的承受者时，谓语的形式叫做被动语态。

二 构成“助动词be+过去分词”主动词be有时态、人称和数的变化，也可以构成否定或疑问句。

不同时态的被动语态例句
The Great Wall is known all over the world.
This city was liberated in 1948.
The matter will be discussed tomorrow.
The question is being discussed at present.
The boy was being operated on when his parents hurried to the hospital.
The bridge has been built this month
The tickets had been sold out before I came to the cinema
He said the book would be returned as soon as he finished it.
三、 用法：

1 需要强调动作的承受者; 2 只知道动作的承受者，不知道谁是动作的执行者；

3 论述科技内容的文体需要强调客观性和科学性。
四、不及物动词或动词短语无被动语态：
appear, die disappear, end (vi. 结束), fail, happen, last, lie, remain, sit, spread, stand
break out, come true, fall asleep, keep silence, lose heart, take place.
　After the fire, very little remained of my house.
比较： rise, fall, happen是不及物动词；raise, seat是及物动词。
　(错) The price has been risen.
　(对) The price has risen.
　(错) The accident was happened last week.　　　
　(对) The accident happened last week.
　(错) The price has raised.　　　　　　　　　　
　(对) The price has been raised.
　(错) Please seat.　　　　　　　　　　　　　　
　(对) Please be seated.
　　要想正确地使用被动语态，就须注意哪些动词是及物的，哪些是不及物的。特别是一词多义的动词往往有两种用法。解决这一问题唯有在学习过程中多留意积累。

五、 不能用于被动语态的及物动词或动词短语：
fit, have, hold, marry, own, wish, cost, notice, watch agree with, arrive at / in, shake hands with, succeed in, suffer from, happen to, take part in, walk into, belong to
This key just fits the lock.
　Your story agrees with what had already been heard.
六、 系动词无被动语态：
appear, be become, fall, feel, get, grow, keep, look, remain, seem, smell, sound, stay, taste, turn
　　1） It sounds good. 2）The steel feels cold

3) The method proved (to be ) effective.

七、 带同源宾语的及物动词，反身代词，相互代词，不能用于被动语态：
die, death, dream, live, life
　　She dreamed a bad dream last night.

八、当宾语是不定式时，很少用于被动语态。
　　 (对) She likes to swim.
　　 (错) To swim is liked by her.

因为英语中通常以物为主语，所以常用被动，这与中文以人为主语不一样