Adding column from SSMS wizard in SQL Server
It only does that if you try and insert the new column between already existing ones.

If you add it to the end of the table it will just do a simple 

 ALTER TABLE T ADD some_column INT


The reason SSMS does this is because there is no TSQL syntax to reorder the columns in a table. It is only possible by creating a new table.

I agree that this would be a useful addition. Sometimes it would make sense for the new column to be shown next to an existing column when viewing the table definition (e.g. to have a created column next to a last_modified column or telephone_number grouped with address and email columns). 

But a related Connect Item ALTER TABLE syntax for changing column order is closed as "won't fix". The only workaround apart from rebuilding the table would be to create a view with your desired column order.