为什么是a lot of exercise而不是a lots of exercises?

这应该要看语境,判断这里的exercise是什么意思.
exercise作为名词,
（1）译为“练习”、“操”时是可数名词.如 There are many exercises in the book.
We should do eye exercises.
（2）译为“锻炼”时是不可数名词.如 Let's take exercise.
另外,你的问题里面,后面的“a lots of”是打错了吧.a lot of = lots of.
所以a lot of exercise是指“大量的体育锻炼”,a lot of exercises是指“大量的练习（题）”
来看个例题：
They want to be basketball players.They get ___ every day.
A.lot of exercise
B.a lots of exercises
C.a lot of exercise
D.a lot of exercises
应该选C.因为这里应该是指体育锻炼.而不是特指的体操之类的活动.