Because all the Generations are in different places, countries and are far from each other, correct? And one person cannot build all that throughout the whole Pokemon world. Look here for more information on the PC Box System.
Bill's Pokémon Storage System (as seen in Pokémon Red, Blue, and
Yellow, Pokémon Gold, Silver, and Crystal, and Pokémon HeartGold and
SoulSilver) is a Pokémon Storage System used in both regions of Kanto
and Johto. In Generation I and Generation II, Bill's system can store
20 Pokémon per box, while there are 12 boxes in Generation I and 14 in
Generation II. (In the Japanese versions, 30 Pokémon can be stored per
box, and there are 8 boxes in Generation I and 9 in Generation II.)
Generation II also adds the ability to rename the boxes for easier
organization. However, each time a box is changed, the game must be
saved. If a box becomes full, the player must change boxes or no more
Pokémon can be captured. In Generation II, Bill will contact the
player via Pokégear when a box has become full. In HeartGold and
SoulSilver, the Pokémon Storage System uses the Nintendo DS's touch
screen and otherwise operates in the same manner as Bebe's system does
in Generation IV. In Generation I only, Pokémon sent to Bill's PC are
not restored to full health.
Lanette's Pokémon Storage System (as seen in Pokémon Ruby and Sapphire) is an entirely new Pokémon Storage System, with a graphical
user interface and the capacity to store up to 420 Pokémon in 14
boxes, each of which can hold 30 Pokémon. An added feature is usage of
the start and select button, which respectively move the cursor to go
to the box title and highlight the cursor, giving strict movement of
Pokémon. When a box becomes full, Pokémon will be moved to the next
available box eliminating the worry of running out of space.
Brigette's Pokémon Storage System (as seen in Pokémon Box Ruby & Sapphire) is based on Lanette's Pokémon Storage System, but with
greatly expanded capacity and an improved interface; it can hold up to
1500 Pokémon, and allows the user to move many Pokémon at once.
Lanette's Pokémon Storage System (upgraded by Bill, as seen in Pokémon FireRed and LeafGreen and Pokémon Emerald) adds the ability to
manage held items directly from the PC. The amount of boxes stayed the
same and each holds 30 Pokémon. Bebe's Pokémon Storage System (as seen
in Pokémon Diamond, Pearl, and Platinum) is the same as Lanette's
system, but the number of boxes has been increased from 14 to 18, for
a maximum capacity of 540. It was created with help from Bill. It is
possibly an upgraded version of Bill's modification of Lanette's
system. In Platinum, six more wallpaper designs were added. When the Y
button is pressed, the hand turns yellow instead of white, allowing
players to simply pick up and drop Pokémon in necessary slots (as
opposed to confirming this action every time the A button is pressed).
Each box holds 30 Pokémon. Amanita's Pokémon Storage System (as seen
in Pokémon Black and White, and Pokémon Black 2 and White 2) has the
player start out with eight boxes capable of holding 240 Pokémon
total. When all 8 boxes have at least one Pokémon in them, the storage
capacity will be increased to 16 boxes, holding 480 Pokémon. When all
16 of those boxes have at least one Pokémon in them, the capacity will
be increased again to a maximum of 24 boxes, or 720 Pokémon. This
allows for quicker save times early on, as save data for
non-"unlocked" boxes does not need to be calculated, solving a common
complaint about Generation IV's long save times. Each box still holds
30 Pokémon. The other boxes are also shown on the right, as opposed to
the top as they were in HeartGold and SoulSilver. Box names cannot be
any censored words; in Black and White only, Box names cannot contain
more than four numerical characters.
I know it's long :P Hope I helped!