evolution - Protein in fruits

Why do fruits have such a low protein content (with a few exceptions) ? Don't seeds need protein while growing up? In comparison, the egg of a hen contains lots of protein, used to make a chick.

Answer

I don't have time to document this enough to make it a good answer but it's too long for a comment so I'll post it anyway.

A big difference between plants and animals is how they get their food, that implies they have different elements to work with. Proteins are made of amino acids, which contain nitrogen, meaning to make proteins you need lots of nitrogen. Plants get their energy from converting CO2 to carbohydrates; this is a plentiful source of carbon, but they can't get nitrogen that way. There are two ways of getting nitrogen: from the ground, and from converting atmospheric nitrogen to organic nitrogen, a process so energy-intensive that only bacteria evolved the ability to do it and only some plants use those bacteria (basically feeding them some of their hard-won carbohydrates "in exchange for" the organic nitrogen compounds).

Animals on the other hand get their food from plants, which contain all the carbon they might need and contain much higher concentrations of nitrogen than you find in the ground (the plant basically went to a lot of trouble to get all that nitrogen from a large volume of ground into its comparatively small body, or to convert atmospheric nitrogen into organic, useable nitrogen), or from animals which contain even higher concentrations of nitrogen. Note by the way that some herbivorous animals are limited by lack of nitrogen as well; I'm thinking of pollinating insects which can struggle to live off of sugary nectar alone and also need to eat some of the protein-rich pollen even though that's much less advantageous from the point of view of the plant (sugar being very easy to make for the plant but proteins much less so).

Obviously animals who eat other animals don't need to worry about nitrogen intake much at all, other than making sure they get food in general, because the animals they're eating will contain pretty much the same carbon:nitrogen ratio they themselves contain.

tl;dr: the egg contains more proteins than the seed because the chicken that made the egg ate a whole lot of seeds, and all the protein in those seeds ended up concentrated in that one egg.