Plant babies.

[Flibert]

Why can't a plant make new seeds on its own if it has both Male and Female sex cells?

[gen2]

Many plants CAN fertilise them selves to make seeds. In fact in dandelions this is the most common occurrence. However, there is generally an advantage in cross pollination because that introduces genetic variation and the ability to potentially survive unforseen adverse conditions. To achieve cross pollination, some plants go to the extent of having separate male and female flowers - even separate male and female plants. More usually the one flower has both male parts (stamens) and female parts (carpels) but they mature at different times (or are physically separated) to minimise self fertilisation. Some plants use self fertilisation as a last resort so if cross pollination has not occurred, the stamens grow/curl until they touch the stigma or vice versa.