There are at least two sides two this:
1) The contracts may have some access restrictions so that only certain addresses can perform certain administrative tasks. So you may not be able to use the same smart contract. If the smart contract is also open source then you can duplicate it and use that.
2) Many products in the market are not so much about the actual labor which goes into producing the product but about the marketing and the brand. (Think Coca-Cola for example.) In the case of Dapps trust issues may also play a role - you have to trust the frontend and you'd rather use a known trusted company's frontend than an unnamed company's. And even if the contract is open source only 1% of users are able to understand it (probably even less) and only 1% of those are interested in it - most just want to trust the company that their stuff does what it's supposed to do.
Of course you can also have a product only in smart contracts (think of MakerDAO) and let others do the frontends.