Folks not following directions.
Sad but true.
All they hear: "you do not need CO2 to have a nice plant tank".
So they do not address the substrate, the fish load/type and the lack of water changes/plant balance, most do not put enough plant biomass from day one.
Others use improper plants, flow, too much light/not enough light etc, others toss soil like it does no harm.
Algae is not an issue in such tanks if you follow directions.
All of the directions...........
Same with CO2 tanks but it's got more things going on and folks slack and cannot keep up with things. Even if algae does grow, it's at a snail's pace and can pruned off/away etc.
Regards,
Tom Barr