Paul talked about how God is very significant -- even for those who don't have a concept of God. He talked about his attributes can be seen in creation, and even with modern science factored in, there are various pieces of evidence of God (even though we may not necessarily see God). Reasons to Believe pointed out those evidence from science in this article. However, people (past and present) ignored God -- even people who knew that God exists.
Although people knew God, they "did not glorify Him as God [...] but became futile in their thoughts" (1:21). In fact, "their foolish hearts were darkened" (1:21). Basically, the people did not "give credit" to God but rather screwed around in sinful actions. We know that proper credit is an important concept because in the world of academics, plagiarism is a very big problem.
In particular, Paul talked about how people "changed the glory of the incorruptible God into an image made like corruptible man" (1:23) -- even when claiming to be wise (1:22). God is incorruptible and the creator of the entire universe, so using an average bird or animal as a representation kinda downsizes him. Just like how improper credit is considered plagiarism in the academic world, improper credit to God isn't really good. In particular, improper credit can result in downplaying God (which could provide an excuse to ignore God), or it can result in portraying God as endorser of actions that are completely out of character.