Zero similarly doesn't exist in older number systems. Even they learned the errors of their ways, or at least succumbed to natural selection pressures in their own way.
Dipoles have non-uniform directivity. It would make a lousy basis of reference.
???
Yet it's the principal basis of reference that's used in the real world of antenna performance measurement. dBi only has relevance in marketing.
Is the radiation pattern of a dipole uniform in all directions? If not, which single gain value would you use for as the base reference gain? You have this completely backwards. Marketing benefits from a single figure of merit, some "gain" value over a dipole or some other value. It makes sense to you and me as some indication of its performance in presumably the forward direction.
Antennas are passive devices. Forward gain comes from accepting energy from one direction while rejecting energy from other directions. (Simplisticly speaking, since nits count.) By itself, it says very little about the antenna. The single figure of merit is meaningful mostly for marketing. A picture here is worth a few thousand words. Have a look at
If you don't understand the concepts of antennas (dipoles in particular) and lobes, there is no point in discussing this further.
Go buy those antennas with 2.1 dB more gain (*calculated* against...thin air) than the identical product of their competitor (*measured* against a real life antenna)r. You're a perfect subject for their market.
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