This is a good question actually, and I have and this asked many times from my patients that had came and ask me this over the counter.

There are a few reasons why manufacturers do this, but I'll mention 2 here:

1. For symptoms of the cold and flu, your symptoms may vary from one symptom to another, even during the course of the cold or flu itself. e.g. it can start off with a runny nose and then progress into a cough or a sore throat due to nasal drip.
So having a combination product with multiple active ingredients will cover all bases of symptoms, and help you get through the cold itself. Those drugs are a there in your system just in case those symptoms occur, so it's not a waste per say.

2. Cost.
It costs less to have tablets/caps/liquid manufactured with multiple active ingredients, than having one active ingredient per product. This cost would be reflected for the consumer who is buying this as well.
In regards to safety, there has safety and efficacy research put into products before they are placed on the market, and this is regulated by the FDA.
Products are tested for their interactions as well. So it's reasonable to say that if they're already on the market, they would have underwent scrutiny and testing.

Hope this helps.

Source. I'm a pharmacist for 5 years.