They do it so that you’ll carry over your positive impressions with the products you’ve used, to the new products they want to sell you. You like the Apple Mac, so you think you’ll like the Apple iPhone.
But Twitter just has the one product and it’ll always have just the one product. They’re not making a second product, ever. There’s nothing to transfer a favorable impression to. So what’s the “value” of Twitter as a brand, distinct from Twitter as an app? All Twitter is is an app.