"Have a lot of people ask me" — Why causative here?

Your first sentence simply means that

I come across a lot of people who ask me whether it's possible to achieve their goals.

Or

There are a lot of people who ask me whether it's possible to achieve their goals.

Or again

My experience is that a lot of people ask me whether it's possible to achieve their goals.

This use of have may be quite common in practice, but it is definitely not causative.

As we all know

I bet more than 95% of adults stopped on the street in native English-speaking countries would say no to the question "do you know about causatives in English grammar?".

why causative here?

It's not a causative. Dictionary.com and wiktionary.org show 26 different definitions for the verb "have". There are many alternatives to the causative form of the word.

In this case:

To experience, go through, undergo:
We had a hard year last year, with the locust swarms and all that.
He had surgery on his hip yesterday.
I'm having the time of my life!

Also similar to this:

Used to state the existence or presence of someone in a specified relationship with the subject.
I have two sisters.
She doesn't have any friends.
I have a really mean boss.

And what is the difference if I say:

A lot of people ask me if it's possible [...]

Approximately the same. No difference.