Why do people still use mp3?

In a lot of cases, pure ignorance.

In others, convenience.

For portable use, you can get more media in a given space and 320Kbit MP3 has fairly minimal loss of quality, as long as it is created with a high quality encoder such as LAME; some encoders are abysmal.

Many people use bluetooth audio devices anyway - and that uses lossy compression, so the difference between 320K MP3 and FLAC etc. is pretty much lost regardless.

Re. the other file types you mention:

AAC is also lossy compression.

OGG is only a container type like MKV and the content may be compressed with lossy codecs - or may be such as FLAC, "ogg" gives no clues as to which.

It is astonishing to see that while people would play a 40 Gigabytes FullHD or UHD 4K movie (about 2 hours length ). 

But when listen to song and music, they don't mind it is only 40 Megabytes compressed audio (about 10 to 12 songs per album, 45 minutes length ).

(for 2 hours audio song/music, that is about 107 Megabytes)

How big is 40 Gigabytes? it is 40,000 Megabytes. 

107 Megabytes is only about 0.27% of 40 Gigabytes.

Sounds fine to me...

Habits are hard to break.

Most people aren't as concerned about sound quality

as they are about having a huge library of tracks at their immediate command.

freedom and convenient

most people already have a huge collection of mp3, they dont want to buy new music just for the sake of having new file format. 

they may not want to upgrade or might not be able to afford it

It is more of a brand name than a file format. Many people who use MP3 players use the wav or WMA lossless format for their songs.

The other day I saw a guy walking still using a Walkman. The one that uses Tapes.

People aren't buying hifi systems. Music is played on mobile phones mostly and listened to with small earpods. So it's not like it would make any difference.