Visitor Visa Refusal for my Parents and Sister

My parents and sister applied for a UK visitor's visa to enable them to attend my university graduation ceremony. I am a British citizen since 2011 and have been legally resident in the UK since 2003. This was their 1st UK visa application. The following were included as supporting information:
(a) Invitation letter from University inviting my parents and sister to the graduation ceremony.
(b) Supporting letter from me inviting them to attend my graduation ceremony and indicating that I will host them during their stay and will pay for the expenses they incur during their stay. We had applied for duration of 1 month 23 days (Mid November to Early Jan).
(c) I also indicated that I financially support my parents and sister every year and provided remittance slips showing 3 No. bank transfers into their account over 1 year. I indicated that I support them to the tune of £7,000 per year. In the application form, I had stated £5000 income for my father from me, £1000 for my mother and £1000 for my sister from me. My parents are in their 70s and retired and my sister is in her mid-40s and currently not working.
(d)Being self employed, I also included my accountant's letter stating my finances and 6 months bank statements of savings accounts.
(e) My parents also included their bank statement for 9 months that showed 2 No. bank transfers from my account.
(f) They live in a rented flat for 13 years of which they included their rent receipts for the past 6 months.
g) My parents included their marriage certificate and my sister her divorce certificate as proof of marital status.

All three of them got a refusal with the following identical reasons that the documents do not demonstrate regular payments and that they do not have any assets/investments and since all 3 of them are travelling, they do not demonstrate significant ties to show an intention to leave the UK at the end of the visa.

I can attach the decision here if someone can guide me on how to attach an image or PDF.

Can someone please help me determine where we went wrong? Even though I have been in the UK for 14 years, they have never been interested in visiting the UK except recently to participate in my graduation ceremony. Many thanks.

They have not proved to the ECO that they have strong ties to their home country. The fact that you support all 3 family members financially for their day to day living, would likely have gone against them. Also the fact that you have been here for so long and they have not visited you once is not a good sign.

The ECO likely feels they are not genuine visitors and won't leave the UK as they are financially dependent on you so what reason do they have to go back.

Thank you CR001. Is there a way to successfully obtain a visitor visa for them as I would like them to attend my graduation ceremony?