关于As+形容词+名词+AS

在章振邦新编高级英语语法教程里，有如下描述:

1) I haven’t seen as cold a car as this for years=I haven’t seen a car as cold as this for years.

2) I can’t drink as sweet coffee as this=I can’t drink coffee as sweet as this.

3) I can carry parcels as heavy as this (不可说…as heavy parcels as this.

对于上述的的描述,我有点疑问，最初我认为是因为原级比较结构as….as中的第一个As是副词，所以后面必须加形容词,类似How/So/too/that/等的用法.所以对上述例子1和例子3我都可以理解,但是为什么在例子中，当比较项修饰不可数名词是可以用I can’t drink as sweet coffee as this这种描述,而名词变成复数却不可以说I can carry as heavy parcels as this？这背后的逻辑是啥？