It is just a custom. I always use 很少 in the last part of the sentence, like 我的钱很少，我的行李很少 or use with 的 is both ok 很少的钱，很少的行李(but is a little bit awkward) but for 很多，it can be put before the noun, with 的，or in the last part of the sentence I think it is more flexible.

it is just a habit.we can also use 我的朋友很多=我有很多的朋友

这应该是习惯问题吧，中国人习惯这么用。如果要用的话，我有很少的朋友，这样也说得通。

我有很多朋友means i have a lot of friends. 我有很少朋友means I am a poor man, l have few friends.