Why can't he be Tom? Is it to add a air of mystery,interest and suspence?

Because he doesn't want to be just any old TOM, Dick or Harry !! Boom - Boom

It's the only thing interesting about him in any way, so you might as well let him have it.

Maybe short for Thomas?

Hangover from Anglo-Saxon/Germanic þ?? Maybe we are mispronouncing it?

As it is usually short for Thomas, perhaps more Toms should be Thom in terms of spelling?

And what would an American make of it given that they pronounce Anthony without the hard 't' in the middle...?

It's most of the Toms who are wrong.

Same reason Thom York is a Thom I guess