Can someone please explain to me why the leader is always considered to be heavier than the rest of thr line. In case of a snag, the leader will not break and it can take 50%" of the line if one has to break it??

Leader is not always heavier than the main line. In most cases the leader will be about the same or lighter than the main line.

A few examples...Fly fishing. The main line is weighted and very thick. So the leader needs to be thin and light or it simply will not work. Or using braid. Again the leader is usually lighter than the main line.

There are of course times when you might need the leader to be heavier.

To avoid line loss when snagged you could use a weaker knot or weaker swivel. Say your main line is 20lb then use a 15lb swivel to connect the leader. Most on-line knot web sites list good knots so it might take some searching to find a suitable weaker knot that you are happy with,

The simple way to avoid snag line loss would be to use lighter leader. Fluorocarbon leader is still going to be tougher than your main line even if it is lighter.