Doing so seems to complicate the matter of Spirituality, in my opinion.
I disagree, though it does depend on how you understand spirituality. There are a few issues faced by monotheism, particularly when you have an omnimax god, that just aren't relevant to polytheism.
The problem of evil is a classic example here. A monotheist who believes God is omniscient, omnipotent and omnibenevolent has to contend with the issue of why their deity allows evil to exist. A polytheist doesn't usually have that problem as it's rare for a god to be entirely good. Those few who are entirely (or almost entirely) good aren't generally considered omnipotent.
I suppose the question for you is, do you prefer one God or many Gods?
Many gods for me, especially those gods who can be seen as synonymous with forces of nature. It makes the most sense to me.
Also, why do you believe polytheism exists/existed
That's a question I'm just not able to answer fully, particularly on an internet forum. I can only give my rough understanding.
It seems that polytheism evolved from the animism of our hunter-gatherer ancestors. They began with the notion that the world around them possessed some form of spirit or life-force. Trees, rocks, rivers and so on were seen as having souls of some form just as humans and animals did.
From that initial concept, they gradually added complexity and character to these spirits. They also began to add complexity and character to more abstract concepts such as death, love and war. These spirits were incorporated into mythologies* as powerful beings with their own personalities, history and goals. They became gods.
*An important note here: We have to be very careful about how we interpret mythology. There's a tendency for people to assume our ancestors took it all as literal truth. That's not necessarily the case. The extent to which these stories were taken literally seems to vary wildly between cultures and among people within a given culture.
To illustrate this, look at how people view the Bible. Some take it entirely literally, some view much of it as metaphorical. Ancient polytheists weren't much different.