.I think time__________________(spend) in watching TV is too long.为什么写Spent,

I think time spent in watching TV is too long.意思：我想,（被）花在看电视上的时间太长了.
句子分析：
1.I think 为主句
2.time spent in watching TV is too long 为宾语从句
其中,time spent in watching TV 为主语,它可写为
time spent in watching TV = time to be spent in watching TV
即
spent = to be spent