And since when does 1500 hours guarantee you that? If you're a CFI you will not have gained much actual night IMC, you will not have ever seen icing conditions, all you'll have done is local practice in fair weather. I can go rent a C-152 for 1500 hours and fly only in day VMC in the pattern and be ready for the airlines, right?
1) Renting an airplane for 1500 hours does not equate to being a CFI for 1500 hours. When you rent, you're flying when you want to and at your convenience. As a CFI, it is a job on which is relied for income. And although you still make go/no-go decisions, it is still not without the added pressure that flying is your livelihood.
2) Flying a rental for building hours does not equate to a CFI instructing to build hours. Not only are you teaching and demonstrating, but you're constantly looking for that fine line between how far to let the student "screw up" (we learn from those screw-ups) in order to facilitate learning and not bending the metal. And believe me, the little stick time that I got from instructing was definitely made up for by the instances of having to prevent bending metal. On top of all that, you're still gaining decision making and CRM skills that wouldn't be gained by just renting.
So, no. Renting isn't the same as instructing.
Overall, this is a good rule since it is better at encouraging a career progression approach to an airliner/corporate jet etc. vs pay to play. Sure, there will be those that slip through the cracks (like those resourced enough to rent an airplane for 1500 hours). But there are cracks in any system. However, without this rule, it encourages aspiring pilots to go through the zero to hero programs.