Mass killers would just use bombs if they couldn't use guns.
1. You need knowledge to make a bomb, so no knowledge, no bomb. Not the same for a gun
2. Internet searches for bomb making recipes are monitored by the government. They interrupt terrorist events this way. They won't get everyone, but they get the majority. Not the same situation at all for guns.
3. The chemicals for bomb making (mass casualty bombs) are monitored by the govt. They interrupt terrrorist events this way. They won't get everyone, but they get the majority. Not the same situation at all for a gun.
4. Equipment: you should have extra apparatus to do the mixing. You do not want your chemicals mixing prematurely.
5. Storage. You need much better storage for the chemicals for your bomb. Them accidentally mixing, will cause all sorts of issues to your storage facility.
Danger to Offender
6. Missing the chemicals in an explosive is dangerous to beginners, picking up a gun (with just the most modest of knowledge) is not
7. Turning a bomb from an explosive into a bomb (ie within a container, to create an exmplosion that is dangerous to more than just the person right next to it) adds more danger....the danger to yourself, from your own gun, remains constant
8. Weight. You think suicide vests are easy? Those are from expert, practised bomb makers. You average beginner (we're talking about your first time mass killer, which just about all the mass shooters have been), has no chance of making something so small. So it's going to be weighty, if they want a mass killing
9. Bulk. See above.
10. Transport - parts one and two are why vehicle bombs are so popular. And the car adds to the explosive impact. Still, it's likely to be difficult, and dangerous for the bomb maker to get the bomb to the vehicle.
11. Suicide is one option - but many mass killers don't want to die.
11a. A remote detonator is not easy. Searches for it would be monitored much more closely by the govt.
12. Purchases of them would require licences, with very tight control over those licences, and over the sale of them.
13. Making them would not be easy,
14. Installing them (ie connecting them to the home made bomb) so that it is ensured to work, would not be easy
15. Bomb makers don't always want to be identified. Driving a bomb in a car is a good way to be identified, whether you go the hire vehicle route or otherwise.
16. It takes a LOT longer to make a bomb, and plan an attack, than it does to pick up and use a gun.
17. That time allows those who would do mass killings based on recent build up of emotions...if their chosen thought was a massing killing...to cool down, and change their mind. It allows the govt time to notice them, and catch them.
There's a reason even terrorists in Western countries have moved away from bombs towards guns and vehicles.
So no, there isn't just problems, but gaping holes in your nonsense 'they would just use bombs'.
Some (compared to mass shootings) very few might, as they have in the past. Any other view is quite delusional. There's not a shred of logical thought or evidence backing your thought. And remember, I'm not talking about 'might' use a bomb in place of guns...but the nonsense thought that all, or most (you suggest all, or most), or even half, or even a quarter, or even 1/10th.... of mass shooters would use bombs if they didn't have access to guns.
And no, Iraq and the ME are not usable examples for you. They have practised bomb makers from years of wars, and the multitude of terrorist cells. The very, very vast majority of your mass shooters in the U.S. will never have that prolonged practice at bomb making. Nor access to all the devices needed. Nor the oversight on chemicals faced in Western countries. nor the monitoring of internet searches like Western countries face (and yes, I'm aware of TOR and its associated tools to hide searches) etc.