Because you'll loose half a career's worth of inflationary salary rises that way. Also, women might work part time after having children which would skew the average annual salary down. Over a 40 year career, just from inflation alone, you'd be getting about half your final salary that way, even ignoring any increases later on from being better qualified or taking on more responsibility.

Mind you, in the UK, defined benefits pension schemes are very rare nowadays, but where they exist they are defined as a percentage of the final year salary with that company, so the highest 2 year thing seems a bit weird to me but for a different reason.