Why do i have to grind to progress to the story, and why do i have to pay ingame currency if i want to build new weapons and warframes, i had to wait for a month just to get a free weapon slot so i can progress trought the game

Why do i have to grind to progress to the story, and why do i have to pay ingame currency if i want to build new weapons and warframes, i had to wait for a month just to get a free weapon slot so i can progress trought the game