Why is the word 'NAV' only used in context of funds and not securities? A stock can also have a 'NAV' right?
i.e. --> (Assets - liabilities of the underlying company)/ # of shares?
The (assets - liabilities)/#shares of a company is its book value, and that number is included in their reports.
It's easy for a fund to release the net asset value on a daily basis because all of its assets (stocks, bonds, and cash) are given values every day by the market. It's also necessary to have a real time value for a fund as it will be bought and sold every day.
A company can't really do the same thing as it will have much more diverse assets - real estate, cars, inventory, goodwill, etc. The real time value of those assets doesn't have the same meaning as a fund; those assets are used to earn cash, while a fund's business is only to maximize its net asset value.
Nobody tracks a single company's net assets on a daily basis, and stock prices are almost never derived directly from their assets (otherwise there would be no concept of 'growth stocks'). Stocks trade on the presumed current value of future positive cash flow, not on the value of their assets alone.
Funds are totally different. They own nothing but stocks and are valued on the basis on the value of those stocks. (Commodity funds and closed funds muddy the picture somewhat, but basically a fund's only business is owning very liquid assets, not using their assets to produce wealth the way companies do.) A fund has no meaning other than the direct value of its assets.
Even companies which own and exploit large assets, like resource companies, are far more complicated than funds: e.g. gold mining or oil extracting companies derive most of their value from their physical holdings, but those holdings value depends on the moving price and assumed future price of the commodity and also on the operations (efficiency of extraction etc.) Still different from a fund which only owns very liquid assets.