Near as I can tell, Nelson wants to leave a legacy so he is long remembered, and building a lot of temples is one of the ways he has chosen to make his mark.
From what I understand, tithing doesn't go directly to anything (I forget the official source). It is put somewhere to collect interest for 3 years, then used. And when the Church says, "Tithing didn't pay for this. It was investment income," that actually means that it was paid for using the interest on the tithes.
Interesting story in the New Testament. The parable of the talents. I'm pretty sure everyone knows it. When the guy who buried the money gave it back to his master, the master was ticked off that he hadn't at least put it in the bank to gather interest, so that he (the master, the Lord) could have had an increase. This tells me that God owns all of it, including the interest. It isn't a way to slide between the rules. It isn't a way to buy stock in companies nor to build malls (let alone high-end malls).
Also, my understanding is that tithing is for the poor, not to make a multi-billion dollar corporation richer (I used to believe the lies that we were a poor Church and that we had unpaid clergy all the way up to and including the President of it).