Do VHDL for loops only allow incrementing by 1?
The only possibilities available for an index of integer type are to increment by 1:

for i in 1 to 10 loop
    ...
end loop


...or decrement by 1:

for i in 10 downto 1 loop
    ...
end loop


You can use other ways of generating the discrete-range, such as:

for i in Input_Signal'Range


As an aside, there's also a for ... generate, which is similar except that the body of the loop contains concurrent statements, where the body of a for ... loop contains sequential statements.