Why laser is not used to melt/soften material in polyjet?
The question in the title references a specific printer technology, while the question in the body of the post is more generic and is the one being answered here.

There are 3D printers which use lasers to cure liquid resin in a manner similar to MSLA 3D printers. The vat of resin sits atop a transparent panel, under which a laser assembly resides. The computer directs the laser over the surface of the panel in a manner associated with each layer of the print. Once the layer is completed, the bed is lifted, peeling the print from the base of the vat and then lowered again.

There are some printers which apply the laser to the surface of the liquid but the results are the same. The resin is photosensitive and cures when the laser is applied.

Additionally, there are SLS 3D printers. Selective Laser Sintering uses a laser applied to the surface of a nylon powder (one example) which sinters the nylon causing it to bond to adjacent particles. When a layer is completed, the bed of powder is lowered and another layer of powder is applied. The laser again "draws" the layer in the powder and the process continues.

This method requires compensation for shrinking, as the nylon powder changes dimensions as it is melted. The bed is also pre-heated prior to printing in order to reduce the amount of energy required by the laser to sinter the design.