I don't know any tradition that will tell you you can't study it.
But, let's make a comparison between what you call 'esoteric' schools (the term usually refers to Vajrayana)
and say, medical school.
You can go through all of the various steps and study and necessary prerequisites and then enter medical school
and then you have to study and practice, and take many tests, and finally, maybe, you will graduate pre-med.
And after that, perhaps you will go back and study surgery.
And perhaps some day you will even cut open a person's chest while they are still alive,
and remove their heart while it is still beating, and replace it with another one.
That is a big responsibility.
So, until that time comes, no, you cannot just go get a book from Amazon.com and be a heart surgeon
and nobody who is a surgeon or a surgical instructor is going to teach you.
So, the secret isn't that you can't get in,
but there are steps to understanding, and you have to develop an understanding of the context,
what mandalas are, what visualization practices are, and so forth, otherwise it's just nonsense
or, if you don't work through some basic meditation and other preliminary practices, you might just attach all the symbolism and trappings and rituals to all of the emotional and mental baggage you have right now, and then what you end up with is a kind of oriental-decorated neurosis, based on a mistaken understanding, and then and go off on a weird psycho power ego trip and hurt yourself or someone else.
The other reason some teachings are considered 'secret' is because they cannot be expressed in a way that someone who is not familiar with it will understand, just as a heart surgeon might talk to another surgeon using all sorts of terms that only they understand. Furthermore, the transformation in your outlook and understanding of the nature of things, and knowing the essence of your own mind, is something that a teacher can point out to you, but only you can truly experience.
Aside from that, if you want to study it, it is open to you
but I don't know that it has ever been free.
But, definitely less expensive than medical school.
Profile Picture: "The Foaming Monk"
The Chinese characters are Fo (buddha) and Ming (bright). The image is of a student of Buddhism, who, imagining himself to be a monk, and not understanding the true meaning of the words takes the sound of the words literally. Likewise, People on web forums sometime seem to be foaming at the mouth. Original painting by P.Volker /used by permission.