@smooth Honestly that's the wrong view. Consider the following...

1. Stocks, the way most people invest, i.e. on the market are also a speculative wager. Unless you happen to buy pre-ipo, you aren't investing unless your plan is to gather dividends. But if you're betting on selling after an increase in the base price of the stock, then that's just speculation.

2. Almost all companies keep a smallish float on the market and then keep a substantial amount of treasury stock, i.e. stock that isn't for sale. This stock is held within the company. It might be pledged as collateral on a loan, but it isn't liquid.

3. The vast majority of stocks are penny stocks and OTC stocks, as a result they tend to be highly illiquid. So to are most angel and venture offerings.

4. In the case of venture and angel capital, there is almost always a vesting period, i.e. a period wherein you are not allowed to sell your stake. This period is often years. This exists so that you don't end up undercutting future fundraising efforts by the entity you have invested in thereby depriving them of needed capital.

5. Cryptocurrencies almost without exception are not stocks. Stocks confer an ownership right and voting rights in the underlaying organization. They confer a set of well defined and understood rights called Investor Rights and those rights give you a say in the way the company is run. Cryptocurrencies are commodities in almost all cases.

6. Commodities such as cryptocurrencies follow a different set of rules than stocks. The similarity of their public markets aside, commodities have a production schedule and the purpose of purchasing commodities on the open market is to provide liquidity for the producers to meet short and medium range expenses until they can deliver the contracted for goods.

Steem is unique in cryptocurrencies. Ownership of steem itself is exactly like any other commodity crypto. You buy it, you're supporting the witnesses that spend the time and effort to produce it. However steem as a commodity has no obvious use case at the moment. Transfers are free, so even transfer fees are not a use case as they are in the case of BTC, LTC, ETH and nearly all the others.

Yet steem does have one use case in that it can be converted to something which conveys the ability to have a say in the day to day running of the underlaying network. That "something" as you may have guessed is SP. There is a reason that the underlaying software calls "powering up", "vesting". Notice it's vesting and not investing. You are vesting, meaning to create an entitlement to a privilege or a right in this case that right is the right to direct where a proportion of the daily pool is going to go.

I shouldn't have to explain the difference here, but for those just stumbling in and wondering what the big deal is, allow me to explain...
SP and Steem are fundamentally different things. When you hold SP you get to direct the location of newly minted steem. In exchange for that privilege you are agreeing to remove a chunk of steem from the open market. Doing so increases the price of steem and gives those who must sell in order to pay bills, for instance the witnesses, a chance to receive a better price.

If you can just freely convert back from sp to steem, the net effect is that there is both a lot more steem on the market, thereby driving down the market price for everyone and you are also undermining the system because large chunks of steem being sold on the cheap can powered up by bad actors. Those bad actors can cause a ruckus, potentially scaring away people who really do want to invest and those bad actors are incentivized to do so because lower steem price means more cheap steem to power up and cause even more havoc. If it takes 90 days for everyone, bad actors included to cash out that at least limits the damage. They can't just power down and dump, they have to go slow and this fact, limits the incentive for damage and also the amount of cheap steem they can dump.