Securing a visit visa is notoriously problematic for many reasons.
Not least because approval or refusal of the visa is principally based on the discretion of the ECO.
There's no appeal process.

The major challenge is the need by the applicant to prove a non-immigrant intent.
Applicants clearly demonstrate to the satisfaction of the ECO that they do not intend to remain in the UK on a permanent basis.
Immigration law places this burden of proof on the applicant.
In order to grant a visa, the ECO must be satisfied with the applicants strong social, economic and family ties at home that would compel them to leave the UK at the end of the temporary stay.

Unfortunately, in many cases it can be tricky to overcome this presumption.

Reasons to return home and strong/complelling ties differ from country to country, city to city, individual to individual.
Some examples of ties can be a job, a house, social and family relationships, a bank account, possessions etc
The ECO will be looking to see if the aspects of the applicants life that binds them to their home country outweighs those aspects that may cause unwillingness to return home country, or may allow attempts to change or extend their visa status.

The positive aspect of visit visa application is that of it's very low cost.
So nothing to lose and everything to gain.

Quote Originally Posted by cassuk11 View Post
........I'm asking this because I'm very suspicious that they always manage to find some excuse or other to refuse for no valid reason
Any refusal will always be provided with a detailed reason(s) according to immigration rules.
This can actually provide a good opportunity to counter those refusal reasons in any subsequent visa application.

What reason(s) did UKVI give before?

There are so many forum threads on this topic so please do take some time to do a search so that you can fully understand all the needed supporting documentation. (Which can be significant by the way.)

Just to recap, the absolute keys are going to be:-

- Reasons to return home that outweigh reasons to stay
- Financials

Good luck