Two students, 'A' & 'B' are studying under a very genuine Guru 'G'. Due to their upbringing, 'A' trusts the things easily, while 'B' demands lot of time & questions to trust something. One day 'G' imparts an important lesson. Due to inherent faith in 'G', 'A' starts believing straight away, while 'B' does counter questions, does his own research and goes till the root of that teaching.
Husband-Wife, 'H' & 'W' were very loving. Due to some misunderstanding, they got separated and are staying in different cities. After a while, 'H' sees no point in separation and decides to reconcile by offering apology over the phone. 'W' insists that the forgiveness will be granted only if 'H' asks for it in person by meeting.
As seen in the 1st analogy, due to faith, for 'A' the words of 'G' are enough. 'A' implements the lesson quickly. While 'B' has his own set of questions & analysis on the way. Until those things are answered, 'B' cannot genuinely follow the teachings. After all the verification, 'B' accepts the lesson from 'G'. Both 'A' and 'B' are right in their own ways.
In 2nd analogy, 'W' could have accepted efforts of 'H' just by hearing his voice. Things would have mended easily & bring the life on track. Right decision by 'W'. However, she demands 'H' to visit personally to her. She is still right. Because she doesn't want any stone to be unturned & make sure that the reunion is really sincere.
Above analogies explain the similar situation of why "formful" has upper hand over "formless". Both, "believers" and "disbelievers" are right. "Believers of form" attain the supreme quickly due to easy surrender and "Non believers of form" naturally take time to surrender to supreme.
Krishna refers 'I/Me' as the supreme nature [of Brahman]. The true nature of "I/Me" is "unmanifested / formless". But it's not any ordinary "unmanifested". It can neither be thought (chintan) nor be described (being). In a way, this "formless" is beyond whatever "formeless" one can imagine.
BG 2.25 - 'That' is said to be unmanifest, unthinkable, non-distortable. Hence knowing 'that' thus, you shouldn't grieve.
BG 8.20 - But beyond that "unmanifested", there is another eternal "unmanifested" nature; That which doesn't get destroyed, when all beings are destroyed.
BG 13.13 - I shall speak of that which is knowable. Knowing which, one attains immortality. The supreme Brahman (god) dwelling under Me, is beginningless. 'That' is said to be neither existent nor non-existent. (existent = being)
Now, relate the believers of "formless" with above analogies. These "formless believers:
- Either like 'B' with full of doubts, they go into the minutest detail
to understand "formless 'Me'"
- Or like stringent 'W', they want to realize "formless 'Me'" by personal visit!
Hence, Don't question 'My' identity too much. Attain 'Me' by trusting my representatives ("forms").
Where are those representatives?
"I" is purely "Nothing"-ness. But to conceive it, Gita chapter 10 is dedicated to I's representative forms. However if still none of them impresses, then whatever impresses, is 'My' form only. Because 'I' is everywhere.
BG 10.41 — Whichever influential excellence(Sattva) [entity] certainly has prosperity(Shrimad) or energy(Urjita); those you know to be possible as part of 'My' aura only.
If "Nothing" impresses, that means only "I" impresses ("I" = "Nothing"-ness). :-) That's still fine, but that path is painful & time consuming. We ourselves are "forms" (i.e. embodied) of 3 modes and with our forms, we can understand other "forms [of god]" better than complete "formless" which is even beyond thinkable "formless".
Exact answer to your question is replied by Krishna as below:
BG 12.4 - For them who have mind attached to the Unmanifested the struggle is greater; For embodied [beings], the destination to unmanifested is painful.