How is that shirk? I believe Allah is the god and Mohamed is his messenger.
Speech is not enough, you need actions combined with speech to be classed as muslim
Just as you can enter islam you can also leave islam.
Secularism calls to man to govern and follow his desires indulge in individualism and materialism so man becomes almost animalistic no different to wild creatures - Essentially man is worshipping his desires and the laws he creates are in tune with his desires so he is worshipping his nafs which is shirk.
Islam calls to follow Allah's laws and sovereignty belongs to him in his infinite wisdom, we worship Allah so whatever Allah forbids we try to avoid and whatever Allah allows we enjoy
The laws in sharia are not man-made they are the laws of the creator of the universe and following these laws is ibaadah its worship
So secularim is shirk because you are not worshipping Allah but you are worshipping what you want which is your nafs
There are many categories of shirk this is just one branch which is to do with governance.