The answer is simple: it is very inefficient. Do you personally respond well to letters from unknown companies? Imagine that you receive a letter from an online store selling napkins in Russia: everything is beautifully designed, the company tried hard, but why do you need napkins from Russia when you live in Moldova?
Of course, you are not interested in what comes from this online store, and you click on the "This is spam" button or unsubscribe from the mailing list. But in most cases, when people receive irrelevant emails, they still send them to spam.
Subsequently, this will lead to the fact that subsequent mailings will also go to spam, and your domain will be blacklisted. It is known to be extremely difficult to restore the reputation of a domain.
Keep in mind that even if you build a base for email distribution in an honest way, there is still the likelihood of frequent clicking on the "This is spam" button, let alone the purchased base. From here we get the first rule: collect the email database in an honest and legal way. Add a subscription form to your site, give your users an incentive to subscribe by offering them, for example, a discount for it.