why is hydrogen not burned for cars instead of hydrogen fuel cells?

They can, actually there are hundreds of DIY'ers that have converted their engines over to run on hydrogen. But, they have to get their hydrogen from home. Because federal regulations do not allow for public stations for hydrogen. A spark of any sort, even a cold spark can ignite hydrogen. That is why blimps are not as popular as they would have been had the Hindenburg hadn't exploded.

For two reasons, pollution and efficiency.

Insufficient torque is not a problem with electric motors. Torque starts at 0 rpm. In fact the next generation of large trucks may very well be series hybrids where an electric motor replaces the transmission.1 The problem is currently not with the electric drive at all but with supplying the needed electricity.

The electric motor can be over 95% efficient, relatively silent, less than a handful of moving parts, maintenance free, permanently lubricated with no pollution or by products. Where is the problem? Supplying it with electricity in a moving vehicle has a number of solutions. Creating the electricity on board is one. This can be done with photovoltaic cells, (solar car); an ICE, (a series hybrid) or with a fuel cell, (fuel cell electric vehicle, FCEV or shortened to "fuel cell vehicle FCV) Electricity can also be transmitted to the vehicle or stored on the vehicle.

When electricity is stored on a vehicle the weight of the storage device becomes an issue in the vehicle's power to weight ratio. We frequently discuss the "energy density" of the fuel or storage. Chemical bonds can provide a high power density. But when the efficiency is low and has by products as in a gasoline engine it becomes a problem. Part of the pollution of an ICE is from products in the air combining in the hot compressed environment of an ICE. Hydrogen burned in a cylinder would therefore produce pollution that would not exist in a fuel cell.2

The utility of hydrogen is so marginal that maximum efficiency must be achieved. It is inefficient to change electricity into hydrogen when we could just use the electricity.3 It is even less efficient to burn hydrogen than use it in a fuel cell.4

Internal combustion engines are notoriously inefficient. With the low volumetric energy density of hydrogen, you need all the efficiency you can get hence the fuel cell instead of an engine. The weight advantage of hydrogen isn't a major factor with cars but volume certainly is. Also in order to be effective an internal combustion engine running on hydrogen would have to vary it's compression ratio according to the load because hydrogen has a high RON octane rating but a virtually non-existent MON octane rating, this is essentially the same as changing your engine cylinders physical dimensions as you step on the gas pedal. Try re-boring out your cylinders several thousand times a minute, doesn't work too well does it? It's just better to use a fuel cell operating at 95% efficiency followed by an electric motor at 95% efficiency instead of an internal combustion engine which would operate at about 25% or less efficiency. Remember an engine doesn't convert the fuel to kinetic energy, it converts it first to thermal energy and then converts that thermal energy to kinetic energy and that's worse then converting first to electric energy.

Of course, the whole purpose of hydrogen vehicles is to store energy chemically from a clean source such as solar power for use in vehicles. When researchers at Sandia Labs took a look at this, they concluded that it would be just as easy to store that energy chemically as hydrocarbons synthesized from CO2 and H2O and achieve the same results without having to replace vehicles or distribution infrastructure. Hydrogen powered vehicles is more about marketing and selling new vehicles then it is about the environment, if it was just about the environment, it would be easier to just synthesize gasoline from a clean energy source.

Probably not. Aside from all the normal issues of hydrogen production, storage, transfer, & application, there's another thing nobody ever considers. Imagine a future where the highways are packed with hydrogen-burning vehicles. It's rush hour, and thousands of these hydrogen vehicles are whizzing along the roadway - each of them, spewing out a mist of water as by-product of their combustion. Where would all that water go? Sure, some of it would evaporate, and some of it would get blown off the road - but, there's a high likelihood that most of it will stay somewhere within the line of travel of traffic. That includes on windshields obscuring visibility, a mist in the air obscuring visibility, and on the roadway surface making it slippery. And, in the winter it would be terrible; all that water mist would turn into ice; on the roads, on windshields, coating & freezing door handles preventing people from being able to exit their cars when they reach destinations... These are severe drawbacks to the viability of hydrogen as a mainstream fuel of the future.

hydrogen explodes with a much greater force then gasoline, so if that car crashes it will be much more dangerous then gas cars. it is more feasible in planes, because of the rarity of plane crashes.