Brahma's constant dropping of his body does not refer to his actually giving up his body, but rather that he gave up a particular mentality
SB Canto 3
The Lord, who can distinctly see the minds of others, perceived Brahmā's distress and said to him: "Cast off this impure body of yours." Thus commanded by the Lord, Brahmā cast off his body.
The Lord is described here by the word viviktādhyātma-darśanaḥ. If anyone can completely perceive another's distress without doubt, it is the Lord Himself. If someone is in distress and wants to get relief from his friend, sometimes it so happens that his friend does not appreciate the volume of distress he is suffering. But for the Supreme Lord it is not difficult. The Supreme Lord, as Paramātmā, is sitting within the heart of every living entity, and He directly perceives the exact causes of distress. In Bhagavad-gītā the Lord says, sarvasya cāhaṁ hṛdi sanniviṣṭaḥ: (BG 15.15) "I am sitting in everyone's heart, and because of Me one's remembrance and forgetfulness occur." Thus whenever one fully surrenders unto the Supreme Lord, one finds that He is sitting within one's heart. He can give us direction how to get out of dangers or how to approach Him in devotional service. The Lord, however, asked Brahmā to give up his present body because it had created the demoniac principle. According to Śrīdhara Svāmī, Brahmā's constant dropping of his body does not refer to his actually giving up his body. Rather, he suggests that Brahmā gave up a particular mentality. Mind is the subtle body of the living entity. We may sometimes be absorbed in some thought which is sinful, but if we give up the sinful thought, it may be said that we give up the body. Brahmā's mind was not in correct order when he created the demons. It must have been full of passion because the entire creation was passionate; therefore such passionate sons were born. It follows that any father and mother should also be careful while begetting children. The mental condition of a child depends upon the mental status of his parents at the time he is conceived. According to the Vedic system, therefore, the garbhādhāna-saṁskāra, or the ceremony for giving birth to a child, is observed. Before begetting a child, one has to sanctify his perplexed mind. When the parents engage their minds in the lotus feet of the Lord and in such a state the child is born, naturally good devotee children come; when the society is full of such good population, there is no trouble from demoniac mentalities.