朗文词典上be/lie at the root of sth理解中at为什么没有

朗文词典说 be/lie at the root of sth (=be the cause of something)是某事物的根本原因,例句：

1)Allergies are at the root of a lot of health problems. 过敏是许多健康问题的根源。

2)The love of money is the root of all evil . 贪财是万恶之源。

为什么句1）are at the root...有at，而句2）却没有了at, 不能理解？

1)Allergies are at the root of a lot of health problems.

Allergies are the root of the most common problems we see in dogs and cats

2)The love of money is the root of all evil .

The love of money is at the root of all evil.

两者可以互换用，都是比喻性质。各自的例句谷歌上“正规”页面上都蛮多。

这句的at不可少

But behold, the axe is laid at the root of the tree; every tree, therefore, that has not produced good fruit is cut down and falls into the fire.

mengxin Z -

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