Question

why do some plants creep on the ground​

Answer 1

In some plants, the stems are not strong enough to give support to the plants. Such plants cannot stand on their own. These plants either creep on the ground or take support from any other things like fences, walls of buildings, etc.

Answer 2

The plants you're talking about, are called creepers, and they need support as they do not have a rigid stem.

I've answered whatever I know :)

Hope this answers your question!