This is not a sentence, the main verb phrase is missing.

I take it that the assumption was mentioned in the sentence(s) before. I would read this as follows: Because we're dealing with the problem (and (think that we) know everything about it), we jump to the assumption which was uttered before. Simply is an adverb, it belongs to make and means as much as easy or quick. to simply make an assumption = to make a quick assumption or an easy assumption

This is not a complete sentence. In order for me, or someone else to help you, we are going to need more context. Where did you see the sentence? What other sentences were around it? The sentence needs another clause. For example, it might be that the sentence goes something like "We make the assumption that simply because we're dealing with a problem, there must be a solution." Without context it's impossible to say. The sentence is incomplete. The phrase "that simply because" has a literal meaning. It means there is an easy to understand or simple reason for something.