Material score in analysis report

I found the following utility that seems to do the task in a very straightforward way:

http://www.lutanho.net/pgn/pgn2fen.html

So I ask, is it hard to code this? If so, why is it software programs don't offer this feature in their exporting options?

Check the program pgn-extract (https://www.cs.kent.ac.uk/~djb/pgn-extract). It has what you probably need. Here I copy a fragment of its help:

Include a position evaluation after each move (--evaluation)

The --evaluation argument causes a comment to be appended to every move, which contains an evaluation of the position immediately following that move. The default evaluation is a simplified version of Shannon's board evaluation. In this case, the evaluation is the difference between the value of White's position and Black's, where the value of a position is a weighted sum of the pieces plus a multiplier (0.1) applied to the number of available moves for that player.

I see this primarily as being a hook for people who wish to embed their own evaluations in the output. See the evaluate function in apply.c if you wish to write your own.

However, an alternative approach to annotating games with engine analysis would be to consider using something like my UCI-analyser that can pass suitably formatted PGN files to a UCI-compatible engine. See, for instance, the section on obtaining annotated output in PGN format.

pgn-extract: A Portable Game Notation (PGN) Manipulator for Chess Games Version 19-04 by David J. Barnes (@kentdjb)