Why two diode join cant act as transistor?

No, it is not possible because in transistor the depletion layers formed in Emitter-Base Junction & Collector-Base Junction are penetrable by both current carriers but in this case of two diodes; the formed depletion region are not penetrable for current carriers (hole &electron). Also, a transistor works only because the base layer is very thin. You won't get that thin layer between emitter and collector just by connecting two diodes together. This thin base layer places the Emitter and Collector in very close proximity to each other. This allows majority carriers from the emitter to diffuse as minority carriers through the base into the depletion region of the base-collector junction, where the strong electric field collects them. In other words the emitter/base current flow draws some of the barrier charge away from the collector/base junction and allows collector/emitter current to flow across the base using minority carriers. So transistor action is not possible.

But we can make transistor by connecting two diodes and two dependent current sources i.e. Ebers-Moll model of transistor.
This is true only when you want to make the transistor act like a on/off switch, but you cannot make this setup of diodes to act like an amplifier. Whereas the transistor also acts as an amplifier too

A transistor can act as: (1) on/off switch (2) amplifier.

Diode is made up of two layers and one junction.
Transistor is made up of three layers and two junctions.