Third party software - why so necessary for partnerships?

Hello. My wife and I have our own personal tax returns plus our partnership return each year. Our affairs are incredibly simple, Our personal returns can be submmitted directly to HMRC via our personal tax account. And yet our partnership return cannot. We have to buy "software" from a third party in order to submit it. Can anyone shed any light on why, exactly? It's not as if partnership returns are that complicated. I'm exercised about this because, well, it just seems silly, and two, I bought software from a supplier on the HMRC approved list which turned out not to be fit for purpose - it had bugs and it just didn't work. We had to wait for it to be debugged. Very annoying. So - come on HMRC, why not just allow partnerships to submit tax information direct, like personal returns?

Hi Mike Johnson,
The HMRC online tax return, does not support certain types of tax return. 
The guidance advises  "You cannot file a Self Assessment tax return online: for a partnership, for a trust or estate, if you lived abroad as a non-resident, to report multiple ‘chargeable gains’, for example from life insurance, if you get income from a trust, you’re a Lloyd’s underwriter or a religious minister.
In these circumstances you will need to use commercial software or download other forms instead.
File your Self Assessment tax return online
Thank you.