Because it is very hard, unless you have so much passion for it, do not even try it. Building a startup is like jumping off the cliff without anything, building your wings while you are in the air and getting your wings fully done, so you can fly before hitting the ground so you won't crash onto the ground and die.
If you can build the wings before jumping off the cliff, then your chances of success are much higher, so that way you have the wings, have time to test the wings, make sure it is working, working well, may be it doesn't have to work 100% perfectly, at least you have a reasonable wing that you can rely on.
Also doing a startup because it sounds exciting is a bad reason. The good reason is you have an idea that is solving a good problem that many people have and people are willing to pay for it, so you are making people's lives better and also making money in the process.
Here is one way to do it to be successful,
1 - Join an incubator
2 - Thoroughly vet your idea - do customer validation - interview potential customers, gather metrics, will they pay for such a product? How much they would pay? etc
3 - Then build an MVP show it to customers - gather metrics - talk to atleast 100 potential customers if your idea is a consumer idea, if it is B2B talk to at least 30 potential customers
4 - Then if you have the money then invest your own money - if not work on fund raising
5 - Work on all the steps above part time
6 - Once you have some funds raised or have some traction with customers & revenue then jump full time -
So now you have your wings that are reasonably okay, you can jump off the cliff
Typically successful founders with product companies had one of these two things
1 - They worked 2 to 3 years part time, developed a reasonable 1st version of the product, won some customers, got some revenue, then went full time with it, then did fund raising and grew from there or grew from the revenue
2 - They did a Phd in a subject, where they owned some intellectual property, had issued patents - leveraged those patents, raised money & then went full time with it
Also check out my book Puga Sankara's Supply Chain Blog - that is a must read before spending Hundreds & Thousands of $$$ on your MBA or Masters Degree to get the best ROI or before starting your Entrepreneurial journey.