Direct sales involve handling credit cards (i.e. chargebacks, scams) as well as getting in bed with potentially every tax authority in the world. For example, for every game sold in European country, you need to collect VAT and transfer it to that country's tax authority (currently, there's something like 28 of them?). This also includes filling out forms... Steam takes care of all that for you.
However! Publishers don't do that. Publishers' value proposition is funding the development and doing the marketing for you, so that you can concentrate on making the game. If the game tanks, they eat up the losses (i.e. it's an investment, not a loan). The retail is a separate issue and, while it's handled by the publishers, is still likely to be handled via Steam or other platforms.