Any reasons for 'DO' statement not returning result?

Hi,

I'm just curious about the reasons of the design of 'DO' statement so that
it is not able to return result of the SELECT in its body.

References:
https://www.postgresql.org/docs/current/static/sql-do.html

http://stackoverflow.com/questions/14652477/how-to-perform-a-select-query-in-a-do-block

With some former experience with MsSQL server, where 'complex' script is
executed easily and straightforward without any 'wrapping', like this
dummy-one ...

DECLARE @a int;
DECLARE @b int;
...
select @a + @b as "a+b"

... every time I need to execute some one-time-through-away complex code in
PostgreSQL which returns rowset I'm disappointed - this has to be wrapped
into normal 'temp' function which I have to delete all the time in current
session, thus making an anonymous 'DO' statement use-less in 95% of my
use-cases.

So ... may someone know good reasons for such inconvenient design of 'DO'
statement?

Thanks.