Where are all the multiterminal discrete transistors?
The picture you show is probably from a TTL-style AND gate, where the multi-emitter transistor was used to function as a bunch of diodes (each from B to En), where each emitter could be used to pull the base connector low, thus implementing the AND function.

The reason this was preferred over separate diodes is that a multi-emitter transistor takes up less space on a chip than the corresponding set of diodes. And once you have such multi-emitter transistors on a chip, nicely integrated in a complete AND port, why would you want to build an (inferior) AND port yourself from its components? (If you wanted to do so, using separate diodes would be easier because they are small and cheap.)

A totally different use of muti-emitters is current splitting. With careful manufacturing a transistor can be made so that the current on one emitter is always a fixed part (let's say 0.01) of the current of the other emitter. This is used in OpAmps to create current sources, and in low-drop voltage regulators to implement overcurrent protection without a sense resistor in series with the main current (which would add to the dropout voltage). Again, no separate multi-emitters of this kind are (widely?) available because no-one wants to build something from scratch that is available better and cheaper in one package.