Why Does The Automatic Scripts That Are Generated Have So Much Stuff?

For example if I make a column not null the SQL Script generated.

Removes all indexes, keys, etc...
than creates a temp table
Than moves the data into the temp table
then deletes the old table
renames the temp table
Recreate all the indexes, keys, etc...

Is there a structure reason for it? Like now that the DB is being alerted that the column is not null it wants to re insert all the data so it can re-organize it?

Because I can just write my own script to the effect of

ALTER [TABLE] [COLUMN] PARAM TYPE NOT NULL
one line of script that seemingly does the exact same thing..

So I'm just curious when I change a column to non null why doesn't it just generate a simple one liner?