>Sure, they banned him after he lost the election and could no longer retaliate meaningfully against them, while his opponents were coming into power.
What means of retaliation do you believe Trump would have had against social media platforms while in office? A President doesn't have the legal authority to simply outlaw companies they disagree with, nor do they have a right to a social media account, or to force businesses to accept them as a customer, or carte blanche to violate contracts or terms of service. Sign an executive order making it illegal to ban his accounts? Executive orders aren't laws, and Presidents aren't monarchs.
Meanwhile, as a billionaire celebrity even out of office, Trump remains perfectly capable of suing over his social media bans, which he is currently doing.