Yes owning a business cost money. It's also true in D&D
Unless you have slaves, never feed them, live in a land without taxes and don't produce anything, a business will cost money. Cutting wood from trees and making chairs might seem like a cheap activity but you need tools, room to store wood. Eventually tools break and you need to replace or repair them. The cost increase depending on the business. Managing a tavern (imagine the cost of all those adventurers breaking stuff and starting brawls) will be more expensive than being a flute maker. You also need customers to buy from you and it can be difficult to convince people to buy from you instead of someone else (specially in a world where bargaining is expected for every transaction).
Instead of overcomplicating the process by going into specifics the DMG set a base cost based on the type of business. The base cost covers everything (tools, land, taxes, possible lost/stolen items, salary of skilled and untrained employees etc.) and then you, the business owner walks in.
Unless your business is established and so big and using a really modern infrastructure, you can't just go on carousing every night like Tony Stark and let the NPC employees manage the business. Why? Because when the boss is not around it gets messy. When the owner of a small business is in the tavern drinking and playing dice and the entire town knows about it, it could hurt the business reputation and people don't go to a business where the manager is obviously too busy. Coming from a small town myself, people create relationship with business owners and if the owner is never around, they usually gravitate to a business with a more caring or charismatic owner. Employees become lazy (or outright steal from you) when the boss is not around.
So based on the rules, which abstract a lot for you, if you own a large business and spend a lot of time there (during your downtime) you'll get bonuses to roll for a probable profit. Unless you neglect the business or you are incredibly unlucky, you'll make money. In the end, it can be really lucrative to own a business. It's also a lot of fun and can bring a lot of roleplay and even adventures to the game.