Thanks for bringing up an important topic. As someone who runs a startup in a similar space, I thought I'd share my observations here.
The primary issue I see with most startups trying to solve the problem is that they go after merely the discovery problem. While I wholeheartedly agree that it is a huge issue for several customers, it is not the most rotten thing about this extremely fragmented industry. The dependability factor is what begs of a solution.
One of the worst things event hosts experience is the feeling of insecurity on the day of the event. Will a priest, caterer, decorator, DJ show up? Will he/she do a good job? Will my guests be adequately impressed? Will one of the vendors ask for extra payment and create a scene?
At Eventosaur, we've handled events ranging from baby showers to corporate parties and everything in between. To assume that our customers came to us (especially repeat customers) just so we could connect them with vendors would be naive.
The other edge of the frequency (or lack thereof) sword is the fact that since users don't host these events often and don't deal with vendors regularly, they are easily hoodwinked. Think about it this way, if you couldn't buy a pair of jeans online and only bought jeans once in 4 years: You'd go into stores without knowing what price to expect. If you were in a market where bargaining was allowed, your negotiations would be biased by the starting price quoted. MBA-esque folks call this 'anchoring' I believe. Whereas with a platform that helps you book all the services at once, the chances of being taken for a ride are lower.
To the folks who commented about customers going around the platform, there will always be a leakage problem. People will always find ways to spend less money. However, as platforms that aim to solve the problem it is our job to make sure that we educate them about the advantages of trusting us over their own negotiation skills.
A vendor can fool you and despite your social media klout and all that, remain relatively unaffected by your anger. If he angers/tries to fool someone like Anji or me, we'll make sure they never see an order from us ever again.
Like one of the kind contributors here mentioned, there's scope for a lot of in-depth analysis here. Happy to take any questions if you any of you are curious about how this works.
(P.S: I run www.eventosaur.com which currently helps users plan traditional events such as griha praveshams, havans and other poojas. Do check us out and pass on any feedback you may have)