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Since plants have a large number of stomata at different parts of their body, they can get $O_2$ at almost every part of the plant and so, they don't need to transport it throughout the whole body whereas in the case of human beings, they can only intake $O_2$ through their nostrils, but a large number of organs throughout their body require it. So it needs to be transported. And for this reason, blood is present.
Let me know if I'm wrong and what the actual answer is.
In plant cells, O2 enters through stomata or lenticels. Then, the O2 molecules are transported to the cells through the process of diffusion. Stomata and lenticels are found all over the surface of plant body, i.e., leaves, stem and root. Because of this, plants don't need blood like connective tissue to transport O2 and CO2.
