Why do the polls of edge in these days?

Political polling has a lot to answer for.

Starting in what we must now call Year Zero for New UK (2016), they predicted a Remain victory with ease in the Referendum, which was one of the reasons Cameron went for it.

Just the Year before Year -1, they had predicted Ed Milliband would squeak in as Prime Minister.

In 2017 the Tories were due a big majority under May who duly held and election and lost.

There is an outside chance we see this in 2019 too.

But in the last 2 elections Labour has closed the gap at the end of the campaign, according to opinion polls. This time for further back so hopefully too little too late. But why do all the polls gather around a point in unison, especially as often in the past, as above, they have then ALL been wrong.

As competitors in a market, there is no incentive to all have the same results, or else the buyers would reduce the competition in the market to the lowest price offer?

I can't work out why this happens  - any ideas?