A $2 screen guard protects your $200 phone investment. The guard isn't an investment, it's a cheap and disposable item whose only purpose is to minimize damage to the much more expensive product it's attached to. Of course a $200 lavish dinner is not an investment; if you've got $200 to blow on one meal, your threshold for what is a disposable item and what is not is higher than that of an average person.
An "investment" in colloquial usage within the context of retail goods is obtaining something that you need to remain working for a significant period of time. If something is relatively very cheap, the financial barrier to replacing it is low, thus if it breaks you just go get another one. If something is relatively expensive (let's say a $1200 computer which you had to save money for three months to afford) then you can't replace it easily if it breaks, so you purchase carefully and with greater importance placed on long term reliability. That's the only reason the purchase of a consumer good that will only ever depreciate in market value is called "an investment." Contrary to your assertions that "price based definition makes no sense, does not fit reality, and is totally stupid," it does in fact make perfect sense, is based entirely in reality, and is quite correct.