It is money saving, time and causes less problems for software providers.
At the end of every year, HMRC INSIST you order the P60 templates (and other documents like this) from them for printing on top of. This means HMRC have the following costs :-
1. Printing the forms.
2. Packing them up when a company orders some.
3. Cost of postage to send them to the company.
Also remember that a lot of companies order more than they need as spares AND some in case printing fails first time.
Another problem for this is when it comes to printing onto them. Alignment has ALWAYS failed when I have printed mine and often caused wasted sheets. If you let the software providers have the template and integrate it, the entire page could be generated by their software and no alignment problems would exist.