Help understanding Book value and Market value when making investing desicions

I am an incoming IB intern who is trying to brush up on their technicals.

I know and understand the definition of book and market value (ie book and market value of equity). What I don't understand is why the market doesn't consider book value (wholly) when making investment choices and why they indirectly mix together accounting with other valuation methods instead.

In your traditional university stock competition, the main valuation you undertake for a stock is the DCF ie cash flow discounting to get enterprise value, then minus debt and add cash/non op assets (roughly) to get Equity value. Now, I compare that to the market price.

I have a problem with this because its mixing up accounting principles with market perception of value.

Book equity value is just Assets- Debt, and then
Therefore, enterprise value is equity value + debt - non-operating assets (roughly)

Now, what I don't get is how we can use market cap equity value and then use accounting terms to get to enterprise value, or how we can simply use DCF to get enterprise value, then use accounting principles to get equity value.

We are combining methods of how markets wish to value things like enterprise value (DCF) or equity value (market cap), and then mixing accounting principles even tho the accounting view and market perception view are not using the same basis, so how can we mix both LOGICS??????????