Lasers do exist in the story, but they're not widely used in warfare because when a laser makes contact with a holtzmann shield (the personal shields that all of the characters employ) it causes a nuclear reaction that annihilates both parties and everyone in a large radius. This makes them extremely risky to use.

There's a great moment in the book where a character sets a trap by engaging a shield and fleeing the area before an errant enemy laser eventually connects with it and creates a massive explosion to cripple the invading forces.

With that in mind the careless use of lasers in the film on two occasions is puzzling considering that it would be an extremely risky and dangerous tactic in the book's universe.

All this implies that shields and/or "lasers" employ massive amounts of energy. Yet both are man-portable. What gives ?

big worm on sand planet make spice that give prophetic ability. yet this is not how worms work. what gives?

This happens towards the end of Dune. It forces the Sardukar to proceed with extreme caution.

Lasers also attract worms with such intensity they cross territorial boundaries.

A brilliant way to say “yea, they exist, but not really” like the nuclear weapons of today.

Just to expand upon what other people have said not only does it cause a big explosion which usually kills whoever is firing the laser it also looks almost indistinguishable from atomic weapons, which are considered a war crime in the Dune universe. Essentially all the major powers have signed an agreement to all hang up and nuke anyone who nukes anyone.

This means lasers aren't even a good suicide attack strategy because it may lead to your entire house being glassed