Because as in structs padding slack can lead to a surprising amount of overhead.

That's not the case:

"SQLite does not pad or align columns within a row. Everything is tightly packed together using minimal space."

https://sqlite.org/forum/info/06ad7f81fea46401

One reason to reorder columns with SQLite is that if a column is usually null or has the default value, SQLite will not store the column at all if it is at the end of the row. It only saves a couple of bytes per column, but it is a reason to get these columns at the end.

AFAIK position has nothing to do with nulls, a null is a 0 byte in the header and has no payload in the row: https://www.sqlite.org/fileformat.html#record_format

Continue reading that section:

"Missing values at the end of the record are filled in using the default value for the corresponding columns defined in the table schema."

If you have a table with 5 columns and you only insert the first 3 columns (based on create table column order) because the last 2 values are null or default, SQLite will only insert 3 type bytes in the header. However, if the first column (in create table order) is the one you omit, SQLite has to include its type byte, even if the value is null.

I reorder columns all the time for neater readability of "select *" queries.

Not true.