Why are the tying laws not enforced in the US, or Canada, or the many other jurisdictions where it's illegal for a monopoly to tie products together; and why do

Why are the tying laws not enforced in the US, or Canada, or the many other jurisdictions where it's illegal for a monopoly to tie products together; and why does it not apply to the tying of Word and Excel or other apps in the Office suite that use to be sold independently and complete with independent products (Lotus 123, Wordperfect)