Why Brahma is not being worshipped in the way Vishnu and Shiva are being worshipped?

It is all due to misunderstanding the spiritual concepts of Rig Veda.

In Rig Veda, the formless Energy/BRAHMAN was eulogised with various epithets, viz., AGNI, INDRA, SARASWATI, ADITI, VISHNU, etc. 

And, occasionally, Indra, Saraswati, etc, are mentioned as BRAHMA, the formless BRAHMAN/God.

a) Rig Veda 2.12.14 (Indra)

यः सुन्वन्तमवति यः पचन्तं यः शंसन्तं यः शशमानमूती । यस्य ब्रह्म वर्धनं यस्य सोमो यस्येदं राधः स जनास इन्द्रः ॥
yaḥ sunvantam avati yaḥ pacantaṃ yaḥ śaṃsantaṃ yaḥ śaśamānam ūtī | yasya brahma vardhanaṃ yasya somo yasyedaṃ rādhaḥ sa janāsa indraḥ ||

English translation:

Who helps through his help the one pressing soma, the one cooking (an offering), the one praising, the one laboring, whose is the strengthening formulation, is the soma, is this gift – BRAHMAN, o peoples, is Indra.

b) Rig Veda 2.41.18 (Saraswati)

इमा ब्रह्म सरस्वति जुषस्व वाजिनीवति । या ते मन्म गृत्समदा ऋतावरि प्रिया देवेषु जुह्वति ॥
imā brahma sarasvati juṣasva vājinīvati | yā te manma gṛtsamadā ṛtāvari priyā deveṣu juhvati ||

English translation:

Enjoy these sacred formulations here, o BRAHMAN Sarasvatī rich in prize-winning mares, the thoughts dear to the gods that the Gr̥tsamadas pour for you, the embodiment of ṛtam (The cosmic Truth).

c) By the time Ramayana was composed, the formless BRAHMAN/ENERGY of Rig Veda had been personified as a Physical God who is the Lord of all worlds - ब्रह्मा लोकाधिपो यथा.

Please note that eulogisation of Vishnu as Supreme god in Ramaayana is an interpolated version.

तेषामेवं प्रभावानां सर्वेषां दीप्ततेजसाम्।।1.18.34।।

पिता दशरथो हृष्टो ब्रह्मा लोकाधिपो यथा।

All his sons were enriched with knowledge and endowed with all virtues. They were modest, renowned, omniscient and farsighted. Dasaratha rejoiced at the sight of those glorious sons with such faculties and looked like Brahma, Lord of the worlds.

d) By the time Puranas were composed, Brahma, Lord of the worlds was degraded to less powerful God/creator compared to Vishnu or Shiva or Devi.

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That was the interesting tale of the formless BRAHMAN/ENERGY of Rig Veda to less powerful God/creator compared to Vishnu or Shiva or Devi.

The Puranic stories created an impression that BRAHMA is lustful, less powerful.

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I do not think we should be worried about the present status of physical God BRAHMA, if we stick to the concept of the formless BRAHMAN/ENERGY of Rig Veda.

Readers need to think for themselves.