Non-dualism is a state that has to be reached. It is not natural or obvious. The natural state is that of dualism for everyone of us, as we all can see, feel the presence of many all the time.
Even the propounders of Advaita must be feeling "many" all the time except when in the state of Samadhi they felt that eventually everything is one.
The point is non-dualism is not obvious, it's not a foregone conclusion. It takes some good amount of spiritual practice to achieve that stage and that practice comes only through a Guru.
IMO, your doubt has been addressed and dispelled very well as well in the following writings of Swami Vivekananda:
CHAPTER IV THE NEED OF GURU
Every soul is destined to be perfect, and every being, in the end,
will attain the state of perfection. Whatever we are now is the result
of our acts and thoughts in the past; and whatever we shall be in the
future will be the result of what we think and do now. But this, the
shaping of our own destinies, does not preclude our receiving help
from outside; nay, in the vast majority of cases such help is
absolutely necessary. When it comes, the higher powers and
possibilities of the soul are quickened, spiritual life is awakened,
growth is animated, and man becomes holy and perfect in the end.
This quickening impulse cannot be derived from books. The soul can only receive impulses from another soul, and from nothing else. We
may study books all our lives, we may become very intellectual, but in
the end we find that we have not developed at all spiritually. It is
not true that a high order of intellectual development always goes
hand in hand with a proportionate development of the spiritual side in
Man. In studying books we are sometimes deluded into thinking that
thereby we are being spiritually helped; but if we analyse the effect
of the study of books on ourselves, we shall find that at the utmost
it is only our intellect that derives profit from such studies, and
not our inner spirit. This inadequacy of books to quicken spiritual
growth is the reason why, although almost every one of us can speak
most wonderfully on spiritual matters, when it comes to action and the
living of a truly spiritual life, we find ourselves so awfully
deficient. To quicken the spirit, the impulse must come from another
The person from whose soul such impulse comes is called the Guru — the
teacher; and the person to whose soul the impulse is conveyed is
called the Shishya — the student. To convey such an impulse to any
soul, in the first place, the soul from which it proceeds must possess
the power of transmitting it, as it were, to another; and in the
second place, the soul to which it is transmitted must be fit to
receive it. The seed must be a living seed, and the field must be
ready ploughed; and when both these conditions are fulfilled, a
wonderful growth of genuine religion takes place. "The true preacher
of religion has to be of wonderful capabilities, and clever shall his
hearer be" —
Source --- The Complete Works of Swami Vivekananda/Volume
3/Bhakti-Yoga/The Need of Guru
Eventually there is only ONE entity. But at present we can not feel it and that's why we need to practice. And, that practice has to be done only under the guidance of a Guru. When, someone is permanently absolved in that state of oneness, there is nothing like Guru-Shishya etc. But, before that we need the knowledge that flows only through the Guru Sishya Parampara.
Advaita Vedanta, Kashmiri Shaivism all have their respective Guru Paramparas, through which the knowledge of Advaita has descended. So, there is no contradiction at all. Eventually there is only ONE but before that stage is achieved, we need the teaching, we need the teacher and we need to practice.