It is more of a rights issue, while DC is the place that these characters originated, it is important to give credit where credit is due. Some estates or creators will not allow characters to be used or if you license an entity to another brand, they get exclusive rights. Getting access is a process that takes time and cannot be rushed as that can lead to lawsuits.
If Jerry Siegel and Joe Shuster are remembered for anything other than Superman, it is how the comic industry screwed them over and left them poor to their dying days, which led to a movement where creators get paid for their creativity rather than having their ideas stolen to make executives richer.
For an example on licensing, Super Sentai toys cannot be made here because its western counterpart Power Rangers has domain and toy rights exclusive to this region of the world through Hasbro, so Bandai cannot make toys as that would breach the contract agreements.
There might be other factors but it never involves “media company is just being mean”. You sometimes have to look at it from a business perspective or legal one.