Solo is much more difficult if you have mobs on you, and if you're not a veteran player you can die real easily. There are some people who don't want to always be in a party.
There are many good reasons why soloing can be better than being in a party. its a hassle communicating with a group of strangers most of the time when your spiral knight friends aren't online. also they tend to also go afk.
So can you please lower the cost of energy revival when you solo?
When you go solo you agree that you are not working with other people.
One of the core features of this game is teamwork, meaning if you are not working with other people it should be harder because you have less power in your team to fight the inhabitants of the Clockworks. Notice that enemy health does not simply multiply by how many members are in the party, but increases by about 10% for every additional member. However, for every party member with you is one more knight fighting; your combat ability is increased by 100% with every member because every knight is 100% of a knight. Math.
Now for the main issue with parties: working with other people. More specifically, working with idiots. Though there are individuals who know how to work with others without annoying the liquids out of their pants, many players prefer to freelance and do whatever they want whether it helps or hurts the rest of the team. Teams are supposed to work together, right? Exactly. They should, because they are a team, which is why in this particular forum section there have been suggestions to make teamwork more appealing so people want to work together. For example, in my engineering suggestion there are several parts outside of that one post where people using these weapons would have to rely on each other to stay alive; but at the same time, together they are a powerful force. For example, if one person makes a turret then another person can use the repairing tool to keep that turret alive after making some barricades to block melee attacks while another builds a buffing tower to give the turret more damage. Three people, one little base of damage buffs and what not. However, if you only have the one turret with nothing else and the restrictions set as they are that one loner will have problems keeping up in terms of damage. Want to try engineer weapons while soloing? It fails horribly. Want to try engineer weapons with other engineers? Aerodynamic aardvarks, this is amazing.