Why aren't UK visa applicants given a chance to explain themselves before getting a refusal? And does one refusal affect other applications?

Does visa refusal for one category of visa application affect the visa application in another visa category (i.e. Visa refusal in Tier 5 visa affect a visitor visa application)?

Yes, it might, there is no way to predict without seeing all of the relevant information. I took a course in Tier 5 aimed at practitioners and the instructor was saying that a refusal for T'X' would lead to refusals for T'Y' when X > Y. So if a person was refused for a Tier 1 and then tried to get a Tier 2, they would likely have problems. And it goes all the way down the line. So if a person was refused a T4 and then tried to get a T5, it would be difficult to succeed. The rationale is that people who are not genuine applicants will keep trying something new until they succeed. It's a generic 'rule of thumb' and only holds true for people who appear to have secondary agendas once they reach the UK.

A visitor application is at the end of this cascade. So people who have been refused for more senior types of visas will have to prepare their visitor application with extra special diligence. More often than not, their premise will fail because for example genuine visitors do not have a history of failed applications on the economic side (T1, T2, and so on). It's a rule of thumb and not carved in stone and of course there are from time-to-time successful applications. Overall about 90+% of applications world-wide are successful and UKVI likes to keep that figure impressively high.

In cases of visa refusal for visitor visa , why aren't applicants given a chance before a refusal is issued, to clear out any queries arising out of their application?

In fact they are. The general rule is that an ECO will contact the applicant for a missing piece of evidence if he thinks that doing so will save the application and prevent a refusal. If there's lots of missing evidence, or the evidence is uniformly weak, he will not contact the applicant and send a refusal instead. Most of the time if the applicant's premise is weak, their evidence will be weak and the person will get refused. I know there's lots of anecdotes on the net about captious refusals, but unless these anecdotes are accompanied by the actual refusal notice, you cannot rely that the correspondent is presenting the full story.

The Chief Inspector keeps an eye on this and ECO's that refused an application when they should not have (or did not contact the applicant when doing so would have saved the application) will attract "a comment" (that's a bad thing for the ECO and his post) and UKVI gets upset when that happens.

Finally, you asked about interviews. I used to do mock interviews for clients... mostly in Moscow and Peter (and sometimes in remote places like Perm or Tomsk, and sometimes in person, sometimes on the phone). They used to interview all applicants. Nobody liked it (i.e., the British public, including me) and they stopped doing it in 2005. And when they stopped interviewing, the refusal rate went down. In this era, they will extraordinarily interview an applicant, most often for T4, but never for a visitor visa. It's not effective and it gets families and friends in the UK angry when there's a refusal. The solution is to prepare a great application and include all the stuff they need to see.

Related article: How to avoid getting a visa refusal when applying for a UK visitor visa?