The reason is tax related….
Effectively, with having you as a self employed consultant there would be a risk to the company that HMRC may challenge the arrangement and argue that it is an employed relationship rather than self employed. The company could then be hit with an employers national insurance charge and penalties etc.
A popular arrangement for IT consultants and the similar professionals (in particular high profile public sector individuals (but that is another story)) is to set up a company themselves and then have that company engage your former employer for your services.
This doesn’t necessarily remove the risk of HMRC challenge to the arrangement, but it shifts the risk from your former employer to your company.
It may be worth suggesting this kind of arrangement to see if your former employer would consider it. But if they are, I would seriously consider getting some “proper” tax advice.
(PS I am a chartered tax advisor, so would be happy to talk through it with you informally, or indeed formally should you wish)
If you want to contact me grahamDOTbirdATsgllpDOTcoDOTuk
Hope that is of some help!
Graham