Good question. The reason these plastic net pots are used is because they allow some growers to produce the highest-quality plants. Speaking for Costa Farms, the net pots are incredibly helpful for health/quality of the cuttings being shipped from where the mother plants are grown in the Dominican Republic to Miami where the plants are eventually grown out.

Once the cuttings reach Miami and are ready for planting, they're well rooted enough that removal of the net pots increases crop loss, incidence of disease, and lower-quality plants.
I know it's easy to point to it being a question of laziness, but they've tested removing the cuttings from the net pots and have seen the negative results in production.

That all being said, the growers at Costa Farms have been aggressively looking for and testing alternatives to these plastic net pots --- and as soon as they find a solution that doesn't negatively impact crop quality, they'll switch over to it.

One other note: In their testing, they've found it takes a number of years for the net pots to negatively impact plant health in average home conditions. So for the first several years after you purchase your Aglaonema, the net pots shouldn't factor into its overall health.

Hope this makes sense, and I hope it doesn't sound like I'm trying to defend the use of the net pots. I'm just offering an answer to your question of "why."