If you buy an existing business, you can analyse the profit and loss statements and balance sheet of the Company you are buying. This is an advantage because you see firsthand what you are getting in, particular the income streams of the business and the individual expenses of the business.
In addition to this you have a going concern producing revenue, with staff, brand, systems and an infrastructure in place. In essence you are making money from the day you settle.
When you start a business from scratch, you must secure and fit out a premise, build the systems, web site, arrange plant & equipment, hire and train staff- all of this before you begin to earn an income.
Statically most start up businesses struggle to make a profit and in some cases fail within 3 years from commencing. Experience would say that starting up a business require an enormous and of persistence & resilience but above all a bullet proof business plan.