There are too many questions in that. I don’t think most of the answers to your "why" have any convincing scientific or religious validation. They all would be answered only with rational ideas and observations.
Let's scratch them one-by-one…
"Why do only humans have religions?"
Because humans can think and are cognitive. We have a brain (and heart), which is comparatively much powerful than that of the animals'. The things that separate us from the animals also provide us an edge over them, making us a much smarter living creature.
This is why humans have religion.
Besides, should you know, God didn’t exactly give us "religion". It's just a concept made by humans out of social needs, propaganda, personal agendas, and political acuteness. Religion exists because humans have an active brain. Animals do not necessarily have that luxury. So, they likely never got together and created religions for themselves.
This also takes us to your other question:
Do you think even animals, birds and plants too having religious issues?
NO. Animals, birds, and plants do not have religious issues. Because their brains do not function like ours. And besides, their priorities are usually to eat and survive.
(Courtesy: AZ Quotes)
(Think of it, if you're to choose between trying to live and care about religion, you would choose trying to live. It is only in the luxury of our comfortable home and full stomach do we pay heed to all the unnecessary things, including religion.)
Is god biased on humans...if so, why?
Yes, according to the Bible, God is biased towards humans.
I am not sure why?!
Maybe because we're one of His/Her most beautiful creations?
Maybe because we have quite an active brain that makes us very vulnerable. So, it requires more care?
Maybe because we're the biggest threat to each other so we require most of God's attention?
Maybe because we're worse of all so we have to be looked after the most?
It's like that family where that undisciplined child usually has the most of parent's attention vs. the one who is disciplined.
I don’t know. These seem to be the only possible reasons.
Are the others not his creations...
Yes, everything is God's creation.
Why don't these religious Gods ask the animals and birds to follow his book?
Because God never asked even the humans to follow His/Her "book" - whatever that book is. We follow the religious ideas and rules NOT because God has asked us to. We do that because people have asked you to do that!
Again, religion is a man-made thing. The ideas of religion aren't what God wants/wanted… They are what the creators of the religions wanted.
There's a big difference between being religious and believing in God.
(Courtesy: Open The World Blog)
A person who never visits temples and mosques can be the biggest believer in God vs. those (pretentious) fellows who pray five times a day and chant "Jai shree ram". It has never been about what you do in front of the world -- it’s about what you believe in when nobody is looking.
The majority of the world's population that follows religions, they aren’t following God. They are just following the ideas of individuals who created their respective religion. God never asked to be blindfolded in faith -- the makers of religion did. God never asked His/Her creation that has the brain to kill people in Her/His name -- the makers of the religion did.
In short, religion and religious textbooks aren't God-made. They are human-made.
(Courtesy: Just Dial)
God never asked us to follow his "rules". Similarly, He/She never asked animals and every other living creature to follow his "rules".
These answers aren’t based on any scientific and religious proof. They are simply what makes sense the most. They are rational ideas and understanding.
Of course, both overly-religious people and atheists will disagree with most of the answers above. And that's okay. As long as their ideas and opinions aren’t extreme and hurting to anyone, they are entitled to that.