It is because of the aesthetics of aquariums. People enjoy watching fish or other aquatic species and wish to be close to them with no barriers otherwise they feel it is worth spending money on space in order for their bond not to be interrupted. Aquariums need conditions like right temperature, various lightings and filters that are completely different from those in nature, so more than one animal can live freely. As a consequence, there would not only be synthetic plants but also special types of artificial grass as well as some decorative rocks and stones etcetera necessary to make an environment more life-like rather than just plain glass surfaces or plastic tubes containing water which fish can swim around immersed in.