Question

How do Pokemon that can't be found in the wild have a catch rate?

Answer 1

>Approximately, the probability of catching a Pokémon starts with the species' catch rate divided by 255. Then it is multiplied by the following factors
The health of the Pokémon (relative to its full health), which can reduce the probability to ⅓× at full health.
The type of Poké Ball, which can augment by some ×.
Any status ailment of the wild Pokémon, i.e. 2× increase for sleep or freeze, and 1.5× for others

The reason being is that they have a formula, and Pokemon that can't be found in the wild can apply to this formula as well.
You can also ask Game Freak.

Hope This Helped :D

Answer 2

Kinda as a 'just-in-case' case. If you just so HAPPEN to find this Pokemon in the wild (through some in-game glitch or by Gameshark, etc.), you won't be stuck with an uncatchable PKMN or a 100%-catchable one. If there's no data for that, then there'd be "NULL" data added into the formula, which would basically produce a random and potentially dangerous situation.
It all has to do with the coding of a game. Computers have to follow basic instructions, and formulas work better when there is data to input into them - even if the calculation is never used - than IN CASE SOMETHING HAPPENS where this formula gets used unintentionally.