A tank top is not a T-shirt without sleeves. That's a muscle tee, or I-shirt. A tank top has smaller straps over the shoulders, and is cut lower under the arms. Tank tops are also called A-shirts, vests, singlets, wifebeaters, and other names. The wifebeater term may be descriptive of some wretched git who doesn't take time to dress; such as Andy Capp, or Onslow from the TV show "Keeping Up Appearances".
Tank tops are often classified as underwear, usually come in white, and are easily found. They are OK as underwear, but the white ones are not stylish for outerwear. The same is true of a white T; why wear that? Well, people do. It's common.
Colorful tanks have become popular outerwear in warmer climates since about the 80's, and are more often worn by women. Possibly that is because women have a much wider selection of colors, and more styles, such as spaghetti straps and T-backs.
Search the man's casual department and you might find some nice-looking colored tank tops. A guy's wardrobe can include a few great looking tank tops in various colors, but you might have to search.
Colorful tank tops for men are not always easy to find (hm, maybe start a website selling colored tanks). The woman's ones come in various colors, but they often have spaghetti straps or are cut way low in the front, or both, and do not suit a guy.
It's a convenience to wear as little clothing as possible. This is practical. Wear more clothing and there is more washing to do. The more clothing, the smellier it gets.
Anyway, clothing is highly overrated. Shorts and a tank top are comfortable, and decent enough to wear to a summer picnic or to answer the door. I hear knocking.
Update: It was mentioned that a tank top can also be a sleeveless sweater.