Grab Car or Just Grab might be, but Grab Taxi is the metered price with a surcharge. The drivers make more when hired under the taxi booking than they do Grab Car. In fact the amount taken from private drivers of Grab Car is pretty offensive.
The price shown when you book a taxi via Grab is an estimate and in my estimation, always significantly higher than the final price.
Thai taxi costs are cheap - they meter fare has only recently changed after years stagnant at the initial fee. Drive times have not improved, fuel has increased dramatically. When meters were first introduced, fuel was around 8-11 Baht if I remember correctly... There are plenty of dishonest and unscrupulous taxi drivers, but there are also large numbers of honest and hardworking ones. The fares are too low and that is part of, not all, the incentive to lie and cheat for those who are inclined to do so.
Were the fares more in line with the cost of living, the incentive to cheat would be reduced. There will always be those that prey on the newbies in tourist zones, but that needs a different mode of control.