Why don't we use positrons from beta plus radiation as a source of anti-electrons for energy?

The important number is not the efficiency of the conversion of mass to energy. Instead, what you should be looking at is the efficiency of the conversion of the resulting gamma radiation to electric current. This conversion has never been particularly efficient, since generating a usable electric current typically involves giving a large number of electrons each a little bit of energy, while radiation (especially ionizing radiation like gamma rays) tends to give a small number of electrons each a lot of energy. Of course, the latter can lead to the former, but only once the few energetic electrons collide with many other electrons. This may produce a current, with the right setup, but it will also produce heat, as the energy absorbed by the initial electrons is dissipated in the random motion of the electrons around them. The production of heat means that the conversion process is not perfectly efficient.

Beta plus decay is one of the possible decays in radioactive materials. To get just positrons would mean one would have to produce materials that decay into positrons,i.e. spend energy in doing that.

Positron decay is given by various isotopes , which are not found in great reserves so as to be useful in producing energy by the gamma rays given off the annihilation of the positrons on electrons.

Radioactivity has been harnessed in radioisotope thermoelectric generators :

A radioisotope thermoelectric generator, or RTG, uses the fact that radioactive materials (such as plutonium) generate heat as they decay into non-radioactive materials. The heat used is converted into electricity by an array of thermocouples which then power the spacecraft.

We do. Systems that do this are called betavoltaics.