Reason #1: Money is fungible; money designated for one category still impacts your ability to spend in any other category. 

Lets say that the government gives you a "housing allowance" of $1000 each month. It's money that can only be spent on housing; you're not allowed to use it on entertainment, transportation, or even groceries. Okay, fine - but now, because your housing costs are covered/supplemented, you have an extra $1000 of discretionary funds that you can spend each month (money you would have spent on housing, but for the housing allowance).

Reason #2: It's all tithing money in the first place. Where else would the Church's investment money come from?

The Church currently has a massive fund. The money's location, value, liquidity (and even the terminology used to describe it) are subject to a lot of debate, but we all agree that the Church is quite wealthy. That money clearly started as tithing. Tithing is what allowed the Church to dig its way out of the $2m hole in the 1890s. Tithing (and local contributions to building funds, ward activity budgets, etc.) is what allowed the Church to expand and grow in the 1900s. Tithing is still how the Church covers a lot of its operational expenses. Tithing is what gave the Church its nest egg to invest in the first place.