It is said that following World Wide Web Consortium, why do many websites (even some big stations) have many nonstandard places?

  html5, question

For example, div is not closed, img lacks alt, uses html5 obsolete tags and some obsolete attributes such as, some have multiple id duplicate names. These mistakes are obviously avoidable!

Historical reasons: big companies don’t necessarily attach importance to standards, and they don’t pay attention to old codes without problems. After all, businessmen are after profits.
Environmental reasons: Do you really think that big stations are big companies?
Human reason: Many Cheng Xuyuan only work, not love.