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The formation of stars is a captivating process beginning with the gravitational collapse of a cloud of interstellar gas and dust. At the core of this event is the balance between gravitational forces pulling particles inward and internal pressure pushing outward. When the inward pull overcomes the outward pressure, the cloud collapses, leading to the birth of new stars. This process is not uniform across all clouds due to variations in size and density.

Furthermore, the initial conditions such as temperature and angular momentum affect the collapse. The phenomenon known as 'Jeans Instability' underlines that only clouds exceeding a certain mass and size, known as the Jeans mass, will collapse under their own gravity. Smaller clouds tend to remain stable against gravitational collapse, which explains why larger, massive stars are less common; they require significantly more material to begin the process of star formation.

When delving into the realm of galaxies, we find that the mass distribution of interstellar gas and dust clouds, leading to star formation, follows a power-law distribution. This implies that there is a higher prevalence of low-mass regions compared to high-mass ones. Indeed, there are more smaller clouds than massive ones, which naturally leads to a greater number of low to intermediate-mass stars.

Galactic dynamics, including spiral arm density waves and the galactic rotational curve, play a significant role in the accumulation and distribution of these clouds. Moreover, the turbulence within the interstellar medium often leads to cloud fragmentation, favoring the creation of smaller clouds, thereby predisposing the galaxy to birth more smaller stars than supermassive ones.

The lifespans of stars are intrinsically tied to their masses. Massive stars, those with a signficant amount of mass compared to the Sun, have comparatively shorter lifespans due to their rapid rate of nuclear fusion. In their cores, these titans burn through their nuclear fuel at a staggering speed, leading to a rather spectacular but brief existence.

Massive stars exhaust their hydrogen fuel and transition through subsequent stages of burning heavier elements much quicker than their less massive counterparts. As a result, while a star like the Sun has a lifespan of about 10 billion years, a star with ten times the mass of the Sun may only last about 20 million years. This fleeting existence means that we are less likely to observe them during their supermassive phase. Coupled with their rarity due to the initial mass distribution in galaxies, the short lifespans of massive stars add to the impression that supermassive stars are exceptionally uncommon in the observable universe.