Almost all the fruits are acidic in nature, but why? At least, fruits like apples should be neutral right? But although apple is sweet, it is acidic. Why?
Fruits have culinary and botanical meaning. For the former, we call fruits those botanical fruits that are juicy and taste more or less sweet and sour. For the latter, e.g. nuts are non juicy fruits. Tomatoes are fruits botanically, but considered as vegetables for culinary purposes.
Acidic in chemistry context means pH<7, not necessarily tasting acidic/sour. Presence of organic acids ( citric, malic, tartaric acid) causes fruit juice being acidic. Sufficient presence of soluble sugars masks acidic taste by stimulating sensors for sweetness.
Take a juice and dilute it by water. Even if pH raises and objectively it gets less acidic, subjectively it seems more acidic, as sugar concentration gets lower, unmasking sour taste. Acidity decreases slowly by dilution, due higher dissociation of weak acids and due existence of pH buffers. Alkaline fruits would be probably poisonous, full of alkaloids.
The question why there are free acids is rather question for botany. Probably the part of strategy to get seeds distributed by keeping fruits from being attractive until they are ripe and sweet.
In biochemistry, acids are COOH groups, while bases are amines and amides. Nitrogen containing substances are a limited resource in almost any organism. On the other hand, carboxylic groups can almost freely convert to and from sugars.
In regard to fruits:
It looks like a very old kind of co-evolution between the fruit eaters (that spread the seeds) and fruit-producing plants (that pay for the seeds distribution by making edible and tasty fruits).
The high acidic content of the fruit (together with the green color) signals that the fruit has low nutritional value (the seeds are immature and eating the fruit now is not valuable to the plant).
Low acidity, high sugar content, red/orange/yellow color distinct from the green leaves - the fruit is ripe and ready to be eaten (and the seeds are ready to be spread).