For one, it costs more money to dye the plastic or paint/anodize the metal. Everyone except children are going to buy the product based on function or need and companies have determined that the colour has no outsized effect on which one they choose. So it's cheaper to use basic black plastic and unpainted metal.
The other is that for things like cars and furniture there's a resale factor. A car is a big investment, and you want as much money as possible to put towards the purchase of the next one. A bright colour like yellow might make you happy while you own it, but it will be difficult to sell to other people as it's a very loud and ostentatious colour. It's the same with furniture and home architecture. '70s patterns in carpet will reduce the resale value of a house because few people will be willing to adapt their furniture to match.
For corporate environments it's about ease of repurposing the space and to be the least bit disruptive to where they're doing business. It'd cost a lot of money to remodel all the old red steel roof McDonald's if that lot were ever sold. In fact there are plenty of gable roof Pizza Huts that just got repainted after they were sold because remodeling the building cost too much money. McDonald's trend towards grey earth tones using metal and stone is so they can fit into big cities these days, especially in Europe and Asia. The coffeehouse aesthetic that Starbucks pioneered has proven especially attractive in those markets, and other companies are following suit by trying to use that same look to blend into modern five over ones and block highrises.