Maybe; we don't actually know how common Earth-Type planets are. They could be a dime a dozen.
We can make an educated guess.
Yeah, and that educated guess puts earth-type planets as surprisingly common.
Hell, even if they were suitably rare, say one per ten thousand stars, there are enough stars in the galaxy that that still means millions of the damn things.
One of the cool things about a growing population is size alone doesn't make it slow it's grow. If there is room to expand, birthrates rocket up to accommodate. For instance, settlers in America had an average of 6-8 kids in the early years. The same is true in Africa today. Livable land will always be a valuable commodity.
In other words, lots of life-sustaining planets just means a bigger alien population, not a lot of unused habitable planets.
Also if life isn't guaranteed to happen on a earth like planet they will probably prefer one without life so they can more easily create a ecosystem like their own and grow crops they can use.
This is simple enough to remedy, alter the atmosphere artificially to what you need it to be, that'll kill off the majority of unwanted life. Some creatures will evolve to cope, the number depending on just how much the atmosphere changes. Then just begin ecosystem. Much easier then trying to say, start an ecosystem on Mars the same way.
Some things, like the Gravity of the planet would be much harder if impossible to alter.