No doubt, God is formless Paramatma in its highest form, but worship of formless for a common jiva is very difficult. Even the minds of some of the advanced yogis engrossed in Tapas of Brahman like Vishvamitra were distracted by Apsaras or riches of heavens, while several like Buddha, sage Narayana etc., were put to test by nature and Indra, what to expect from the weak minds of a common Jiva. This was also explained in Geeta.
Geeta Chapter 12
Bhagavad Gita 12.1 Arjun inquired: Between those who are steadfastly
devoted to Your personal form and those who worship the formless
Brahman, who do You consider to be more perfect in Yog?
Bhagavad Gita 12.2 The Blessed Lord said: Those who fix their minds on
Me and always engage in My devotion with steadfast faith, I consider
them to be the best yogis.
Bhagavad Gita 12.3 – 12.4 But those who worship the formless aspect of
the Absolute Truth—the imperishable, the indefinable, the unmanifest,
the all-pervading, the unthinkable, the unchanging, the eternal, and
the immoveable—by restraining their senses and being even-minded
everywhere, such persons, engaged in the welfare of all beings, also
Bhagavad Gita 12.5 For those whose minds are attached to the
unmanifest, the path of realization is full of tribulations. Worship
of the unmanifest is exceedingly difficult for embodied beings
If worship of formless Brahman as mentioned in Vedanta on whom Adi Shankara himself has given summaries, was a piece of cake, than why would even God spend time in creating this ever changing yet never-ending material world Maya. Omnipotent God could have shown theories on Vedanta to every jivatma in their dreams giving them Moksha their itself.
Just like a mind-dominated child can easily understand through fantasies and images so does a mind-dominated jiva can easily understand through the world, its images and various Saguna avatars of Nirguna Brahman. Thats why Adi Shankaracharya who summarized Advaita Vedanta, himself composed several prayers for God worshipping like Bhaja Govindam, Saundaraya Lahiri, Bhavani Ashtakam etc.,
But worship of formless, unseen God leads to failure for a novice sadhaka especially in Kaliyuga, age of darkness. For example, Islam forbids from giving any image to God, but look at their mindless killings of innocent idol-worshippers and non-muslims in the name of Jihad. They fail to understand that creator/God is the creation/world, One/God cant be grasped without the help of other/world.
A man can be of gigantic intellect, yet spiritually he may be a baby.
You can verify it this moment. All of you have been taught to believe
in an Omnipresent God. Try to think of it. How few of you can have any
idea of what omnipresence means! If you struggle hard, you will get
something like the idea of the ocean, or of the sky, or of a vast
stretch of green earth, or of a desert. All these are material images,
and so long as you cannot conceive of the abstract as abstract, of the
ideal as the ideal, you will have to resort to these forms, these
material images. It does not make much difference whether these images
are inside or outside the mind. We are all born idolaters, and
idolatry is good, because it is in the nature of man. Who can get
beyond it? Only the perfect man, the God-man. The rest are all
idolaters. So long as we see this universe before us, with its forms
and shapes, we are all idolaters. This is a gigantic symbol we are
worshipping. He who says he is the body is a born idolater. We are
spirit, spirit that has no form or shape, spirit that is infinite, and
not matter. Therefore, anyone who cannot grasp the abstract, who
cannot think of himself as he is, except in and through matter, as the
body, is an idolater. And yet how people fight among themselves,
calling one another idolaters! In other words, each says, his idol is
right, and the others' are wrong. Therefore, we should get rid of
these childish notions.
Two sorts of persons never require any image — the human animal who
never thinks of any religion, and the perfected being who has passed
through these stages. Between these two points all of us require some
sort of ideal, outside and inside. It may be in the form of a departed
human being, or of a living man or woman. This is clinging to
personality and bodies, and is quite natural. We are prone to
concretise. How could we be here if we did not concretise? We are
concreted spirits, and so we find ourselves here on this earth.
Concretisation has brought us here, and it will take us out.